I might have found a race condition. In bluefish-1.3.* there is a tooltip query callback. To find out which text the mouse is over it calls:
gtk_text_view_get_iter_at_position(GTK_TEXT_VIEW(btv), &iter, NULL, bx, by);
This never gave any problems.

Today I implemented a 'Collapse all' feature for the code-block-folding. The 'Collapse all' is is a popup menu that pops up opon a right-click on the document left margin. Code block folding in Bluefish simply adds a GtkTextStyle with the 'hidden' property to the text within the block.

So immediately after the 'Collapse all' is chosen, we have gtk_text_view_get_iter_at_position running to find the visible text position the mouse is over, and we have lots of places where a textstyle with the 'hidden' property is applied to make text invisible. This is why I think this might be a race condition problem. 

Below is the warning and the gdb backtrace:

(bluefish-unstable:3866): Gtk-WARNING **: /build/buildd/gtk+2.0-2.16.1/gtk/gtktextbtree.c:4017: byte index off the end of the line
Gtk-ERROR **: Byte index 533 is off the end of the line aborting...

+ Trace 217792