Thank you for reporting. Confirmed by multiple users for 2.8 on windows and mac. Does not crash in 2.9 but throws warnings.

I've run gimp with --verbose and last line it throws is 
GEGL-gegl-tile-backend-file.c:ERROR:gegl-tile-backend-file.c:957:gegl_tile_backend_file_ensure_exist: assertion failed: (self->i != -1)

Fixed in master and 2.8 branch, thanks.

I regret to say that v2.8.2 for Windows still (or again) has the same problem:

When using cage tool, as soon as the cage is closed, you shortly see the progress circla, then crash. The --verbose output sais,
"**
GEGL-gegl-tile-backend-file.c:ERROR:gegl-tile-backend-file.c:957:gegl_tile_backend_file_ensure_exist: assertion failed: (self->i != -1)"
Crash message on Windows:
"Microsoft Visual C++ Runtime Library. "X" This application has requested the Runtime to terminate in an unusual way. Please contact the application's support team for more information. (OK)"
--
Using Gimp 2.8.2 64bit on Windows 7 (gimp-2.8.2-setup-1.exe loaded from SF.net after Aug 25, 2012).

(In reply to comment #4)
> I regret to say that v2.8.2 for Windows still (or again) has the same problem:
> 
> When using cage tool, as soon as the cage is closed, you shortly see the
> progress circla, then crash. The --verbose output sais,
> "**
> GEGL-gegl-tile-backend-file.c:ERROR:gegl-tile-backend-file.c:957:gegl_tile_backend_file_ensure_exist:
> assertion failed: (self->i != -1)"

GEGL's current version has that assertion at line 959

http://git.gnome.org/browse/gegl/tree/gegl/buffer/gegl-tile-backend-file.c#n959


> Crash message on Windows:
> "Microsoft Visual C++ Runtime Library. "X" This application has requested the
> Runtime to terminate in an unusual way. Please contact the application's
> support team for more information. (OK)"
> --
> Using Gimp 2.8.2 64bit on Windows 7 (gimp-2.8.2-setup-1.exe loaded from SF.net
> after Aug 25, 2012).

and the last commit dates 2012-08-02 (earlier than 08-25)
http://git.gnome.org/browse/gegl/log/gegl/buffer/gegl-tile-backend-file.c

either v2.8.2 ships an old gegl dll, or it is an installation problem.

Why is this bug resolved as FIXED? The assertion still happens.

Hmm, the code has changed quite a bit, let's track
the current crash in bug 688361.

*** Bug 691238 has been marked as a duplicate of this bug. ***

This bug needs to be reopened. Recently I got gimp 2.8.10 for windows from gimp.org and got cage tool crashed the program.
Steps to reproduce:
1. create blank image
2. Using cage tool mark a rectangle are
3. It does initial computation
4. then it instantly (without waiting me to hit "enter" or to drag any nodes) does applying transformations (not sure about right English wording because I'm using localized version)
5. then it crashes. The last thing gimp --verbose says is
====
GEGL-gegl-tile-backend-file.c:ERROR:gegl-tile-backend-file.c:957:gegl_tile_backe
nd_file_ensure_exist: assertion failed: (self->i != -1)
====
Note: when using existing image it crashes after I move one or two nodes, and behavior is still strange: at applies *something* right after cage got established, does not wait for "enter" and while dragging the node lags as hell.

Same story with 2.8.14 on Windows 7 x64: crashes after closing the cage (connecting the last dot to the first one).
Here's the entire output when started with the --verbose switch (in case anyone still gives a fuck).
[quote]
INIT: gimp_load_config
Parsing 'C:\Users\яъ\.gimp-2.8\unitrc'
Parsing 'D:\GIMP 2\etc\gimp\2.0\gimprc'
Parsing 'C:\Users\яъ\.gimp-2.8\gimprc'
gimp_composite: verbose=no
Processor instruction sets: +mmx +sse +sse2 -3dnow -altivec -vis
Adding theme 'Default' (D:\GIMP 2\share\gimp\2.0\themes\Default)
Adding theme 'Small' (D:\GIMP 2\share\gimp\2.0\themes\Small)
Writing 'C:\Users\яъ\.gimp-2.8\themerc'
Trying splash 'C:\Users\яъ\.gimp-2.8\gimp-splash.png' ... failed
Trying splash 'D:\GIMP 2\share\gimp\2.0\images\gimp-splash.png' ... OK
INIT: gimp_initialize
INIT: gimp_real_initialize
INIT: gui_initialize_after_callback
INIT: gimp_restore
Parsing 'C:\Users\яъ\.gimp-2.8\parasiterc'
Loading 'brush factory' data
Loading 'dynamics factory' data
Loading 'pattern factory' data
Loading 'palette factory' data
Loading 'gradient factory' data
Loading fonts
Loading 'tool preset factory' data
Parsing 'C:\Users\яъ\.gimp-2.8\templaterc'
Parsing 'C:\Users\яъ\.gimp-2.8\modulerc'
Loading module 'D:\GIMP 2\lib\gimp\2.0\modules\libcolor-selector-cmyk.dll'
Unloading module 'D:\GIMP 2\lib\gimp\2.0\modules\libcolor-selector-cmyk.dll'
Loading module 'D:\GIMP 2\lib\gimp\2.0\modules\libcolor-selector-water.dll'
Unloading module 'D:\GIMP 2\lib\gimp\2.0\modules\libcolor-selector-water.dll'
Loading module 'D:\GIMP 2\lib\gimp\2.0\modules\libcolor-selector-wheel.dll'
Unloading module 'D:\GIMP 2\lib\gimp\2.0\modules\libcolor-selector-wheel.dll'
Loading module 'D:\GIMP 2\lib\gimp\2.0\modules\libcontroller-dx-dinput.dll'
Unloading module 'D:\GIMP 2\lib\gimp\2.0\modules\libcontroller-dx-dinput.dll'
Loading module 'D:\GIMP 2\lib\gimp\2.0\modules\libdisplay-filter-color-blind.dll'
Unloading module 'D:\GIMP 2\lib\gimp\2.0\modules\libdisplay-filter-color-blind.dll'
Loading module 'D:\GIMP 2\lib\gimp\2.0\modules\libdisplay-filter-gamma.dll'
Unloading module 'D:\GIMP 2\lib\gimp\2.0\modules\libdisplay-filter-gamma.dll'
Loading module 'D:\GIMP 2\lib\gimp\2.0\modules\libdisplay-filter-high-contrast.dll'
Unloading module 'D:\GIMP 2\lib\gimp\2.0\modules\libdisplay-filter-high-contrast.dll'
Loading module 'D:\GIMP 2\lib\gimp\2.0\modules\libdisplay-filter-lcms.dll'
Unloading module 'D:\GIMP 2\lib\gimp\2.0\modules\libdisplay-filter-lcms.dll'
Loading module 'D:\GIMP 2\lib\gimp\2.0\modules\libdisplay-filter-proof.dll'
Unloading module 'D:\GIMP 2\lib\gimp\2.0\modules\libdisplay-filter-proof.dll'
INIT: gui_restore_callback
clipboard: writable pixbuf format: image/png
clipboard: writable pixbuf format: image/bmp
clipboard: writable pixbuf format: image/x-bmp
clipboard: writable pixbuf format: image/x-MS-bmp
clipboard: writable pixbuf format: image/x-icon
clipboard: writable pixbuf format: image/x-ico
clipboard: writable pixbuf format: image/x-win-bitmap
clipboard: writable pixbuf format: image/tiff
clipboard: writable pixbuf format: image/jpeg
Parsing 'D:\GIMP 2\etc\gimp\2.0\sessionrc'
Parsing 'C:\Users\яъ\.gimp-2.8\dockrc'
Parsing 'C:\Users\яъ\.gimp-2.8\toolrc'
Parsing 'C:\Users\яъ\.gimp-2.8\contextrc'
Parsing 'C:\Users\яъ\.gimp-2.8\tool-options\gimp-rect-select-tool'
Parsing 'C:\Users\яъ\.gimp-2.8\tool-options\gimp-ellipse-select-tool'
Parsing 'C:\Users\яъ\.gimp-2.8\tool-options\gimp-free-select-tool'
Parsing 'C:\Users\яъ\.gimp-2.8\tool-options\gimp-fuzzy-select-tool'
Parsing 'C:\Users\яъ\.gimp-2.8\tool-options\gimp-by-color-select-tool'
Parsing 'C:\Users\яъ\.gimp-2.8\tool-options\gimp-iscissors-tool'
Parsing 'C:\Users\яъ\.gimp-2.8\tool-options\gimp-foreground-select-tool'
Parsing 'C:\Users\яъ\.gimp-2.8\tool-options\gimp-vector-tool'
Parsing 'C:\Users\яъ\.gimp-2.8\tool-options\gimp-color-picker-tool'
Parsing 'C:\Users\яъ\.gimp-2.8\tool-options\gimp-zoom-tool'
Parsing 'C:\Users\яъ\.gimp-2.8\tool-options\gimp-measure-tool'
Parsing 'C:\Users\яъ\.gimp-2.8\tool-options\gimp-move-tool'
Parsing 'C:\Users\яъ\.gimp-2.8\tool-options\gimp-align-tool'
Parsing 'C:\Users\яъ\.gimp-2.8\tool-options\gimp-crop-tool'
Parsing 'C:\Users\яъ\.gimp-2.8\tool-options\gimp-rotate-tool'
Parsing 'C:\Users\яъ\.gimp-2.8\tool-options\gimp-scale-tool'
Parsing 'C:\Users\яъ\.gimp-2.8\tool-options\gimp-shear-tool'
Parsing 'C:\Users\яъ\.gimp-2.8\tool-options\gimp-perspective-tool'
Parsing 'C:\Users\яъ\.gimp-2.8\tool-options\gimp-flip-tool'
Parsing 'C:\Users\яъ\.gimp-2.8\tool-options\gimp-cage-tool'
Parsing 'C:\Users\яъ\.gimp-2.8\tool-options\gimp-text-tool'
Parsing 'C:\Users\яъ\.gimp-2.8\tool-options\gimp-bucket-fill-tool'
Parsing 'C:\Users\яъ\.gimp-2.8\tool-options\gimp-blend-tool'
Parsing 'C:\Users\яъ\.gimp-2.8\tool-options\gimp-pencil-tool'
Parsing 'C:\Users\яъ\.gimp-2.8\tool-options\gimp-paintbrush-tool'
Parsing 'C:\Users\яъ\.gimp-2.8\tool-options\gimp-eraser-tool'
Parsing 'C:\Users\яъ\.gimp-2.8\tool-options\gimp-airbrush-tool'
Parsing 'C:\Users\яъ\.gimp-2.8\tool-options\gimp-ink-tool'
Parsing 'C:\Users\яъ\.gimp-2.8\tool-options\gimp-clone-tool'
Parsing 'C:\Users\яъ\.gimp-2.8\tool-options\gimp-heal-tool'
Parsing 'C:\Users\яъ\.gimp-2.8\tool-options\gimp-perspective-clone-tool'
Parsing 'C:\Users\яъ\.gimp-2.8\tool-options\gimp-convolve-tool'
Parsing 'C:\Users\яъ\.gimp-2.8\tool-options\gimp-smudge-tool'
Parsing 'C:\Users\яъ\.gimp-2.8\tool-options\gimp-dodge-burn-tool'
Parsing 'C:\Users\яъ\.gimp-2.8\tool-options\gimp-desaturate-tool'
Parsing 'C:\Users\яъ\.gimp-2.8\tool-options\gimp-color-balance-tool'
Parsing 'C:\Users\яъ\.gimp-2.8\tool-options\gimp-hue-saturation-tool'
Parsing 'C:\Users\яъ\.gimp-2.8\tool-options\gimp-colorize-tool'
Parsing 'C:\Users\яъ\.gimp-2.8\tool-options\gimp-brightness-contrast-tool'
Parsing 'C:\Users\яъ\.gimp-2.8\tool-options\gimp-threshold-tool'
Parsing 'C:\Users\яъ\.gimp-2.8\tool-options\gimp-levels-tool'
Parsing 'C:\Users\яъ\.gimp-2.8\tool-options\gimp-curves-tool'
Parsing 'C:\Users\яъ\.gimp-2.8\tool-options\gimp-posterize-tool'
Parsing 'C:\Users\яъ\.gimp-2.8\tool-options\gimp-gegl-tool'
INIT: gimp_real_restore
Parsing 'C:\Users\яъ\.gimp-2.8\pluginrc'
Querying plug-in: 'D:\GIMP 2\lib\gimp\2.0\plug-ins\pyconsole.py'
Writing 'C:\Users\яъ\.gimp-2.8\pluginrc'
Starting extension: 'extension-script-fu'
INIT: gui_restore_after_callback
Parsing 'C:\Users\яъ\.gimp-2.8\menurc'
Parsing 'C:\Users\яъ\.gimp-2.8\devicerc'
Parsing 'C:\Users\яъ\.gimp-2.8\controllerrc'
Parsing 'C:\Users\яъ\.gimp-2.8\colorrc'
loading menu 'D:\GIMP 2\share\gimp\2.0\menus\image-menu.xml' for /image-menubar
Loading module 'D:\GIMP 2\lib\gimp\2.0\modules\libdisplay-filter-lcms.dll'
**
GEGL-gegl-tile-backend-file.c:ERROR:gegl-tile-backend-file.c:957:gegl_tile_backend_file_ensure_exist
: assertion failed: (self->i != -1)
[/quote]

Does the nightly 2.8 build from http://nightly.darkrefraction.com/gimp/
still crash too?

I think, your problem is cyrillic username "яъ". There was bug with it (https://bugzilla.gnome.org/show_bug.cgi?id=751175).
You should try to use a nightly build of GIMP with the development versions of GEGL and BABL.

this bug is _not_ fixed. 

Gimp 2.8.16-1ubuntu1.1 always freezes with cage tool
Gimp 2.18.18 (from repository ppa:otto-kesselgulasch/gimp) always freezes with cage tool

I use linux mint 18.

veba: Please provide a stack trace of the crash, with debugging symbols enabled.

$ gimp-2.8 --debug-handlers  --stack-trace-mode always

(gimp-2.8:25667): GLib-GObject-WARNING **: g_object_set_valist: object class 'GeglConfig' has no property named 'cache-size'

(gimp-2.8:25667): GEGL-gegl-operation.c-WARNING **: Cannot change name of operation class 0x2238CC0 from "gimp:point-layer-mode" to "gimp:dissolve-mode"

(gimp-2.8:25667): GEGL-gegl-operation.c-WARNING **: Cannot change name of operation class 0x2238100 from "gimp:point-layer-mode" to "gimp:behind-mode"

(gimp-2.8:25667): GEGL-gegl-operation.c-WARNING **: Cannot change name of operation class 0x22384F0 from "gimp:point-layer-mode" to "gimp:multiply-mode"

(gimp-2.8:25667): GEGL-gegl-operation.c-WARNING **: Cannot change name of operation class 0x2239800 from "gimp:point-layer-mode" to "gimp:screen-mode"

(gimp-2.8:25667): GEGL-gegl-operation.c-WARNING **: Cannot change name of operation class 0x2239B30 from "gimp:point-layer-mode" to "gimp:overlay-mode"

(gimp-2.8:25667): GEGL-gegl-operation.c-WARNING **: Cannot change name of operation class 0x2239F60 from "gimp:point-layer-mode" to "gimp:difference-mode"

(gimp-2.8:25667): GEGL-gegl-operation.c-WARNING **: Cannot change name of operation class 0x223A320 from "gimp:point-layer-mode" to "gimp:addition-mode"

(gimp-2.8:25667): GEGL-gegl-operation.c-WARNING **: Cannot change name of operation class 0x223A760 from "gimp:point-layer-mode" to "gimp:subtract-mode"

(gimp-2.8:25667): GEGL-gegl-operation.c-WARNING **: Cannot change name of operation class 0x223AB20 from "gimp:point-layer-mode" to "gimp:darken-only-mode"

(gimp-2.8:25667): GEGL-gegl-operation.c-WARNING **: Cannot change name of operation class 0x223AF70 from "gimp:point-layer-mode" to "gimp:lighten-only-mode"

(gimp-2.8:25667): GEGL-gegl-operation.c-WARNING **: Cannot change name of operation class 0x223B380 from "gimp:point-layer-mode" to "gimp:hue-mode"

(gimp-2.8:25667): GEGL-gegl-operation.c-WARNING **: Cannot change name of operation class 0x223B6B0 from "gimp:point-layer-mode" to "gimp:saturation-mode"

(gimp-2.8:25667): GEGL-gegl-operation.c-WARNING **: Cannot change name of operation class 0x223C080 from "gimp:point-layer-mode" to "gimp:color-mode"

(gimp-2.8:25667): GEGL-gegl-operation.c-WARNING **: Cannot change name of operation class 0x223C3B0 from "gimp:point-layer-mode" to "gimp:value-mode"

(gimp-2.8:25667): GEGL-gegl-operation.c-WARNING **: Cannot change name of operation class 0x223C710 from "gimp:point-layer-mode" to "gimp:divide-mode"

(gimp-2.8:25667): GEGL-gegl-operation.c-WARNING **: Cannot change name of operation class 0x223CB90 from "gimp:point-layer-mode" to "gimp:dodge-mode"

(gimp-2.8:25667): GEGL-gegl-operation.c-WARNING **: Cannot change name of operation class 0x223CF00 from "gimp:point-layer-mode" to "gimp:burn-mode"

(gimp-2.8:25667): GEGL-gegl-operation.c-WARNING **: Cannot change name of operation class 0x223D3A0 from "gimp:point-layer-mode" to "gimp:hardlight-mode"

(gimp-2.8:25667): GEGL-gegl-operation.c-WARNING **: Cannot change name of operation class 0x223D760 from "gimp:point-layer-mode" to "gimp:softlight-mode"

(gimp-2.8:25667): GEGL-gegl-operation.c-WARNING **: Cannot change name of operation class 0x223DBA0 from "gimp:point-layer-mode" to "gimp:grain-extract-mode"

(gimp-2.8:25667): GEGL-gegl-operation.c-WARNING **: Cannot change name of operation class 0x223E000 from "gimp:point-layer-mode" to "gimp:grain-merge-mode"

(gimp-2.8:25667): GEGL-gegl-operation.c-WARNING **: Cannot change name of operation class 0x223E430 from "gimp:point-layer-mode" to "gimp:color-erase-mode"

(gimp-2.8:25667): GEGL-gegl-operation.c-WARNING **: Cannot change name of operation class 0x223E7D0 from "gimp:point-layer-mode" to "gimp:erase-mode"

(gimp-2.8:25667): GEGL-gegl-operation.c-WARNING **: Cannot change name of operation class 0x223EC30 from "gimp:point-layer-mode" to "gimp:replace-mode"

(gimp-2.8:25667): GEGL-gegl-operation.c-WARNING **: Cannot change name of operation class 0x223EFD0 from "gimp:point-layer-mode" to "gimp:anti-erase-mode"

(script-fu:25675): LibGimpBase-WARNING **: script-fu: gimp_wire_read(): error


1. all lines (except the last one) appearing durring only when the gimp starts
2. when I use cage tool and gimp crash, there is no new debug line, no disk or cpu activity
3. while trying to close the gimp window many times cinammon tell me that the gimp is not responding and ask me if I want to force quit or wait the application. I press force quit and the last line (script-fu:25675) appears in the terminal

(In reply to veba from comment #15)
> $ gimp-2.8 --debug-handlers  --stack-trace-mode always
> 
> (gimp-2.8:25667): GLib-GObject-WARNING **: g_object_set_valist: object class
...
> 
> 
> 1. all lines (except the last one) appearing durring only when the gimp
> starts
> 2. when I use cage tool and gimp crash, there is no new debug line, no disk
> or cpu activity
> 3. while trying to close the gimp window many times cinammon tell me that
> the gimp is not responding and ask me if I want to force quit or wait the
> application. I press force quit and the last line (script-fu:25675) appears
> in the terminal

This is apparently a problem in gimp-2.8 distributed by debian/ubuntu and 
derivatives. They're using the wrong version of GEGL.

see 

https://bugzilla.gnome.org/show_bug.cgi?id=770848

it is anyway a different bug than this one.

Please, report it to your distribution bug tracker.