Test case:

#include <gtk/gtk.h>

int
main (int argc, char** argv)
{
    GtkWidget *view;
    GtkWidget *window;
    GtkTextBuffer* buffer;

    gtk_init (&argc, &argv);

    view = gtk_text_view_new ();
    window = gtk_window_new (GTK_WINDOW_TOPLEVEL);


/*      buffer = gtk_text_view_get_buffer (GTK_TEXT_VIEW (view)); */
/*      gtk_text_buffer_set_text (buffer, "0123456789\0", -1); */

    buffer = gtk_text_buffer_new (NULL);
    gtk_text_buffer_set_text (buffer, "0123456789\0", -1);
    gtk_text_view_set_buffer (GTK_TEXT_VIEW(view), buffer);

    gtk_container_add (GTK_CONTAINER (window), view);
    gtk_widget_show_all (window);

    gtk_main();
}


How to reproduce.
1. Compile and run the program.
2. Set the corsor to any position except start and end positions.
3. Press 'Delete' or 'Backspace' button.

The program dies producing this output:
1 lines in tree 0x8064190
   0| line: 0x8063f58 parent: 0x8063f30 next: 0x8063f70
     segment: 0x806d6a8 type: mark bytes: 0 chars: 0
       left mark `(null)' visible: 0 not_deleteable: 0
     segment: 0x806d650 type: mark bytes: 0 chars: 0
       right mark `gtk_drag_target' visible: 0 not_deleteable: 0
     segment: 0x8133fd8 type: character bytes: 7 chars: 7
       `0123456'
     segment: 0x8133990 type: mark bytes: 0 chars: 0
       left mark `(null)' visible: 0 not_deleteable: 0
     segment: 0x81339b8 type: character bytes: 1 chars: 1
       `7'
     segment: 0x81339d0 type: mark bytes: 0 chars: 0
       right mark `(null)' visible: 0 not_deleteable: 0
     segment: 0x8064288 type: mark bytes: 0 chars: 0
       right mark `insert' visible: 1 not_deleteable: 1
     segment: 0x80642f0 type: mark bytes: 0 chars: 0
       right mark `selection_bound' visible: 0 not_deleteable: 1
     segment: 0x806d630 type: character bytes: 3 chars: 3
       `89
'
   1| line: 0x8063f70 parent: 0x8063f30 next: (nil)
     segment: 0x8063fa0 type: character bytes: 1 chars: 1
       `
'
=================== Tag information
  (no tags in the tree)
=================== Tree nodes
node 0x8063f30 level 0 children 2 lines 2 chars 12
  line 0x8063f58 chars 11 bytes 11
   left mark `(null)' visible: 0
   right mark `gtk_drag_target' visible: 0
   chars `0123456'...
   left mark `(null)' visible: 0
   chars `7'...
   right mark `(null)' visible: 0
   right mark `insert' visible: 1
   right mark `selection_bound' visible: 0
   chars `89\'...
  line 0x8063f70 chars 1 bytes 1
   chars `\'...

test (pid:26538): Gtk-ERROR **: file gtktextiter.c: line 2021
(_gtk_text_iter_forward_indexable_segment): assertion failed:
(gtk_text_iter_is_end (iter))
aborting...
Trace/breakpoint trap


Important! If you uncomment the commented strings in the program source
code and comment the next three strings (i.e. change the buffer
initialization) the bug in not reproducible.


The bug looks very similar to #64883 but probably is not its duplicate.