Today I decided to start organising my photos, so I installed Debian's f-spot package (version 0.4.4-1 from Debian Unstable, running on ppc64). After adding some photos, I pretty quickly discovered a bug similar to the above:

Steps to reproduce:
1. Start f-spot from a terminal.
2. Hit F11 to enter full screen mode with the first selected photo.
3. Move the mouse to the bottom of the screen so that the toolbar appears.
4. Click the "Exit fullscreen" button.

Expected behaviour:
- The fullscreen window goes away and the regular view returns.

Actual behaviour:
- f-spot disappears entirely. In the terminal where I started f-spot, the following traceback is displayed:

Exception in Gtk# callback delegate
  Note: Applications can use GLib.ExceptionManager.UnhandledException to handle the exception.
System.NullReferenceException: Object reference not set to an instance of an object
  at GLib.Signal.voidObjectCallback (IntPtr handle, IntPtr data) [0x00000] 
   at GLib.Signal.voidObjectCallback(IntPtr , IntPtr )
   at Gtk.Object.gtk_object_destroy(IntPtr )
   at Gtk.Object.gtk_object_destroy(IntPtr )
   at Gtk.Object.Destroy()
   at Gtk.Widget.Destroy()
   at FSpot.FullScreenView.ExitAction(System.Object sender, System.EventArgs args)
   at GLib.Signal.voidObjectCallback(IntPtr handle, IntPtr data)
   at GLib.Signal.voidObjectCallback(IntPtr , IntPtr )
   at Gtk.Application.gtk_main()
   at Gtk.Application.gtk_main()
   at Gtk.Application.Run()
   at Gnome.Program.Run()
   at FSpot.Driver.Main(System.String[] args)

Notes:
- Leaving full-screen mode with F11 or Esc works perfectly.

Extra goo suggested by Debian's reportbug:

-- System Information:
Debian Release: lenny/sid
  APT prefers unstable
  APT policy: (500, 'unstable'), (500, 'stable'), (499, 'experimental')
Architecture: powerpc (ppc64)

Kernel: Linux 2.6.25-2-powerpc64 (SMP w/1 CPU core)
Locale: LANG=en_AU.UTF-8, LC_CTYPE=en_AU.UTF-8 (charmap=UTF-8)
Shell: /bin/sh linked to /bin/bash

Versions of packages f-spot depends on:
ii  dbus                  1.2.1-3            simple interprocess messaging syst
ii  libart-2.0-2          2.3.20-2           Library of functions for 2D graphi
ii  libatk1.0-0           1.22.0-1           The ATK accessibility toolkit
ii  libc6                 2.7-13             GNU C Library: Shared libraries
ii  libcairo2             1.6.4-6            The Cairo 2D vector graphics libra
ii  libexif12             0.6.16-2.1         library to parse EXIF files
ii  libflickrnet2.1.5-cil 25277-6            Flickr.Net API Library
ii  libgconf2.0-cil       2.20.1-1           CLI binding for GConf 2.20
ii  libgl1-mesa-glx [libg 7.0.3-5            A free implementation of the OpenG
ii  libglade2.0-cil       2.12.1-1           CLI binding for the Glade librarie
ii  libglib2.0-0          2.16.5-1           The GLib library of C routines
ii  libglib2.0-cil        2.12.1-1           CLI binding for the GLib utility l
ii  libglitz-glx1         0.5.6-1            Glitz OpenGL library GLX backend
ii  libglitz1             0.5.6-1            Glitz OpenGL image compositing lib
ii  libglu1-mesa [libglu1 7.0.3-5            The OpenGL utility library (GLU)
ii  libgnome-keyring1.0-c 1.0.0~svn.r87622-1 CLI library to access the GNOME Ke
ii  libgnome-vfs2.0-cil   2.20.1-1           CLI binding for GnomeVFS 2.20
ii  libgnome2.0-cil       2.20.1-1           CLI binding for GNOME 2.20
ii  libgnomeui-0          2.20.1.1-1         The GNOME 2 libraries (User Interf
ii  libgnomevfs2-0        1:2.22.0-4         GNOME Virtual File System (runtime
ii  libgphoto2-2          2.4.1-2            gphoto2 digital camera library
ii  libgphoto2-port0      2.4.1-2            gphoto2 digital camera port librar
ii  libgtk2.0-0           2.12.11-3          The GTK+ graphical user interface 
ii  libgtk2.0-cil         2.12.1-1           CLI binding for the GTK+ toolkit 2
ii  libgtkhtml3.16-cil    2.20.1-3           CLI binding for GtkHTML 3.16
ii  libjpeg62             6b-14              The Independent JPEG Group's JPEG 
ii  liblcms1              1.17.dfsg-1        Color management library
ii  libmono-addins-gui0.2 0.3.1-5            GTK# frontend library for Mono.Add
ii  libmono-addins0.2-cil 0.3.1-5            addin framework for extensible CLI
ii  libmono-cairo2.0-cil  1.9.1+dfsg-2       Mono Cairo library
ii  libmono-corlib2.0-cil 1.9.1+dfsg-2       Mono core library (2.0)
ii  libmono-sharpzip2.84- 1.9.1+dfsg-2       Mono SharpZipLib library
ii  libmono-sqlite2.0-cil 1.9.1+dfsg-2       Mono Sqlite library
ii  libmono-system-data2. 1.9.1+dfsg-2       Mono System.Data Library
ii  libmono-system-web2.0 1.9.1+dfsg-2       Mono System.Web Library
ii  libmono-system2.0-cil 1.9.1+dfsg-2       Mono System libraries (2.0)
ii  libmono2.0-cil        1.9.1+dfsg-2       Mono libraries (2.0)
ii  libndesk-dbus-glib1.0 0.4.1-1            CLI implementation of D-Bus (GLib 
ii  libndesk-dbus1.0-cil  0.6.0-1            CLI implementation of D-Bus
ii  libx11-6              2:1.1.4-2          X11 client-side library
ii  libxcomposite1        1:0.4.0-3          X11 Composite extension library
ii  mono-runtime          1.9.1+dfsg-2       Mono runtime

Versions of packages f-spot recommends:
ii  dbus-x11                      1.2.1-3    simple interprocess messaging syst
ii  dcraw                         8.86-1     decode raw digital camera images
pn  sqlite                        <none>     (no description available)
ii  sqlite3                       3.5.9-3    A command line interface for SQLit

Is this still happening? I can't reproduce it with latest git. (on x86)

I don't have access to that PPC64 machine anymore, but I'm using F-Spot 0.6.1.5 on Ubuntu Karmic these days, and full-screen mode works fine.

Closing, as the code that caused that has been replaced. Feel free to reopen if the issue still occurs.