I got this when using 2.8.x and gtk/glib from HEAD:

==2749== Invalid read of size 1
==2749==    at 0x25CD1BD9: gtk_entry_backspace (gtkentry.c:2539)
==2749==    by 0x26147ECE: IA__g_cclosure_marshal_VOID__VOID (gmarshal.c:77)
==2749==    by 0x26132FF0: g_type_class_meta_marshal (gclosure.c:514)
==2749==    by 0x26132D8C: IA__g_closure_invoke (gclosure.c:437)
==2749==    by 0x26145CC6: signal_emit_unlocked_R (gsignal.c:2480)
==2749==    by 0x26147CD5: IA__g_signal_emitv (gsignal.c:2113)
==2749==    by 0x25C73C9F: gtk_binding_entry_activate (gtkbindings.c:526)
==2749==    by 0x25C74CFD: binding_match_activate (gtkbindings.c:928)
==2749==    by 0x25C74F3E: gtk_bindings_activate_list (gtkbindings.c:1064)
==2749==    by 0x25C7517F: IA__gtk_bindings_activate_event (gtkbindings.c:1139)
==2749==    by 0x25E1B579: gtk_widget_real_key_press_event (gtkwidget.c:3331)
==2749==    by 0x25CD289F: gtk_entry_key_press (gtkentry.c:1825)
==2749==    by 0x25D37613: _gtk_marshal_BOOLEAN__BOXED (gtkmarshalers.c:83)
==2749==    by 0x26132FF0: g_type_class_meta_marshal (gclosure.c:514)
==2749==    by 0x26132D8C: IA__g_closure_invoke (gclosure.c:437)
==2749==    by 0x26145CC6: signal_emit_unlocked_R (gsignal.c:2480)
==2749==    by 0x26147459: IA__g_signal_emit_valist (gsignal.c:2211)
==2749==    by 0x26147973: IA__g_signal_emit (gsignal.c:2245)
==2749==    by 0x25E1B6FB: gtk_widget_event_internal (gtkwidget.c:3587)
==2749==    by 0x25E2AFAD: IA__gtk_window_propagate_key_event (gtkwindow.c:4552)
==2749==  Address 0x2629EFAD is 1 bytes after a block of size 28 alloc'd
==2749==    at 0x259852A8: malloc (vg_replace_malloc.c:130)
==2749==    by 0x261B16D9: IA__g_malloc (gmem.c:137)
==2749==    by 0x260926E4: pango_layout_get_log_attrs (pango-layout.c:937)
==2749==    by 0x25CD1BCF: gtk_entry_backspace (gtkentry.c:2537)
==2749==    by 0x26147ECE: IA__g_cclosure_marshal_VOID__VOID (gmarshal.c:77)
==2749==    by 0x26132FF0: g_type_class_meta_marshal (gclosure.c:514)
==2749==    by 0x26132D8C: IA__g_closure_invoke (gclosure.c:437)
==2749==    by 0x26145CC6: signal_emit_unlocked_R (gsignal.c:2480)
==2749==    by 0x26147CD5: IA__g_signal_emitv (gsignal.c:2113)
==2749==    by 0x25C73C9F: gtk_binding_entry_activate (gtkbindings.c:526)
==2749==    by 0x25C74CFD: binding_match_activate (gtkbindings.c:928)
==2749==    by 0x25C74F3E: gtk_bindings_activate_list (gtkbindings.c:1064)
==2749==    by 0x25C7517F: IA__gtk_bindings_activate_event (gtkbindings.c:1139)
==2749==    by 0x25E1B579: gtk_widget_real_key_press_event (gtkwidget.c:3331)
==2749==    by 0x25CD289F: gtk_entry_key_press (gtkentry.c:1825)
==2749==    by 0x25D37613: _gtk_marshal_BOOLEAN__BOXED (gtkmarshalers.c:83)
==2749==    by 0x26132FF0: g_type_class_meta_marshal (gclosure.c:514)
==2749==    by 0x26132D8C: IA__g_closure_invoke (gclosure.c:437)
==2749==    by 0x26145CC6: signal_emit_unlocked_R (gsignal.c:2480)
==2749==    by 0x26147459: IA__g_signal_emit_valist (gsignal.c:2211)

Could it be a off by one in pango-layout.c 936-937?