Saw this twice today on 11.10 on a VirtualBox VM (32-bit).

Interestingly, the assertion is different both times.

First failure:

(Tomboy:2043): GLib-GObject-CRITICAL **: g_object_set_data: assertion `G_IS_OBJECT (object)' failed
Stacktrace:

  at (wrapper managed-to-native) Tomboy.NoteSpellChecker.gtkspell_detach (intptr) <0xffffffff>
  at Tomboy.NoteSpellChecker.Detach () <0x00083>
  at Tomboy.NoteSpellChecker.OnEnableSpellcheckChanged (object,Tomboy.NotifyEventArgs) <0x00073>
  at (wrapper delegate-invoke) <Module>.invoke_void__this___object_NotifyEventArgs (object,Tomboy.NotifyEventArgs) <0x00072>
  at (wrapper delegate-invoke) <Module>.invoke_void__this___object_NotifyEventArgs (object,Tomboy.NotifyEventArgs) <0x00072>
  at (wrapper delegate-invoke) <Module>.invoke_void__this___object_NotifyEventArgs (object,Tomboy.NotifyEventArgs) <0x00072>
  at (wrapper delegate-invoke) <Module>.invoke_void__this___object_NotifyEventArgs (object,Tomboy.NotifyEventArgs) <0x00072>
  at (wrapper delegate-invoke) <Module>.invoke_void__this___object_NotifyEventArgs (object,Tomboy.NotifyEventArgs) <0x00072>
  at (wrapper delegate-invoke) <Module>.invoke_void__this___object_NotifyEventArgs (object,Tomboy.NotifyEventArgs) <0x00072>
  at (wrapper delegate-invoke) <Module>.invoke_void__this___object_NotifyEventArgs (object,Tomboy.NotifyEventArgs) <0x00072>
  at (wrapper delegate-invoke) <Module>.invoke_void__this___object_NotifyEventArgs (object,Tomboy.NotifyEventArgs) <0x00072>
  at (wrapper delegate-invoke) <Module>.invoke_void__this___object_NotifyEventArgs (object,Tomboy.NotifyEventArgs) <0x00072>
  at (wrapper delegate-invoke) <Module>.invoke_void__this___object_NotifyEventArgs (object,Tomboy.NotifyEventArgs) <0x00072>
  at (wrapper delegate-invoke) <Module>.invoke_void__this___object_NotifyEventArgs (object,Tomboy.NotifyEventArgs) <0xffffffff>
  at Tomboy.Preferences.OnSettingChanged (object,Tomboy.NotifyEventArgs) <0x00025>
  at Tomboy.GConfPreferencesClient/NotifyWrapper.HandleNotify (object,GConf.NotifyEventArgs) <0x0006e>
  at GConf.NotifyWrapper.NotifyCB (intptr,uint,intptr,intptr) <0x0019c>
  at (wrapper native-to-managed) GConf.NotifyWrapper.NotifyCB (intptr,uint,intptr,intptr) <0xffffffff>
  at (wrapper managed-to-native) Gtk.Application.gtk_main () <0xffffffff>
  at Gtk.Application.Run () <0x0000b>
  at Tomboy.GnomeApplication.StartMainLoop () <0x0000b>
  at Tomboy.Application.StartMainLoop () <0x00019>
  at Tomboy.Tomboy.StartTrayIcon () <0x0004b>
  at Tomboy.Tomboy.Main (string[]) <0x002eb>
  at (wrapper runtime-invoke) <Module>.runtime_invoke_void_object (object,intptr,intptr,intptr) <0xffffffff>

Native stacktrace:

	mono() [0x80e126c]
	mono() [0x812046c]
	mono() [0x805fe7d]
	[0xab640c]
	/usr/lib/libenchant.so.1(enchant_broker_free_dict+0x39) [0xaa6ca9]
	/usr/lib/libgtkspell.so.0(+0x41ea) [0x8351ea]
	[0x77995dc]
	[0x576522c]
	[0x249199c]
	[0x24918e7]
	[0x2491913]
	[0x2491913]
	[0x2491913]
	[0x2491913]
	[0x2491913]
	[0x2491913]
	[0x2491913]
	[0x2491913]
	[0x2491913]
	[0x2491913]
	[0x2491686]
	[0x249162f]
	[0x249141d]
	[0x17d5e8]
	/usr/lib/libgconf-2.so.4(+0x16508) [0x8ce5508]

2nd failure:

gtkspell:ERROR:/build/buildd/gtkspell-2.0.16/./gtkspell/gtkspell.c:752:gtkspell_new_attach: assertion failed: (spell == NULL)
Stacktrace:

  at (wrapper managed-to-native) Tomboy.NoteSpellChecker.gtkspell_new_attach (intptr,string,intptr) <0xffffffff>
  at Tomboy.NoteSpellChecker.Attach () <0x00157>
  at Tomboy.NoteSpellChecker.OnEnableSpellcheckChanged (object,Tomboy.NotifyEventArgs) <0x00063>
  at (wrapper delegate-invoke) <Module>.invoke_void__this___object_NotifyEventArgs (object,Tomboy.NotifyEventArgs) <0x00072>
  at (wrapper delegate-invoke) <Module>.invoke_void__this___object_NotifyEventArgs (object,Tomboy.NotifyEventArgs) <0x00072>
  at (wrapper delegate-invoke) <Module>.invoke_void__this___object_NotifyEventArgs (object,Tomboy.NotifyEventArgs) <0xffffffff>
  at Tomboy.Preferences.OnSettingChanged (object,Tomboy.NotifyEventArgs) <0x00025>
  at Tomboy.GConfPreferencesClient/NotifyWrapper.HandleNotify (object,GConf.NotifyEventArgs) <0x0006e>
  at GConf.NotifyWrapper.NotifyCB (intptr,uint,intptr,intptr) <0x0019c>
  at (wrapper native-to-managed) GConf.NotifyWrapper.NotifyCB (intptr,uint,intptr,intptr) <0xffffffff>
  at (wrapper managed-to-native) Gtk.Application.gtk_main () <0xffffffff>
  at Gtk.Application.Run () <0x0000b>
  at Tomboy.GnomeApplication.StartMainLoop () <0x0000b>
  at Tomboy.Application.StartMainLoop () <0x00019>
  at Tomboy.Tomboy.StartTrayIcon () <0x0004b>
  at Tomboy.Tomboy.Main (string[]) <0x002eb>
  at (wrapper runtime-invoke) <Module>.runtime_invoke_void_object (object,intptr,intptr,intptr) <0xffffffff>

Native stacktrace:

	mono() [0x80e126c]
	[0xf8040c]
	[0xf80416]
	/lib/i386-linux-gnu/libc.so.6(gsignal+0x4f) [0xae8c8f]
	/lib/i386-linux-gnu/libc.so.6(abort+0x175) [0xaec2b5]
	/lib/i386-linux-gnu/libglib-2.0.so.0(g_assertion_message+0x15e) [0x2146ce]
	/lib/i386-linux-gnu/libglib-2.0.so.0(+0x68ccf) [0x214ccf]
	/usr/lib/libgtkspell.so.0(gtkspell_new_attach+0xd0) [0xa7b4a0]
	[0xde1ccc]
	[0x6345bf8]
	[0x88835b4]
	[0x888350f]
	[0x888353b]
	[0x888353b]
	[0x88832ae]
	[0x8883257]
	[0x8883045]
	[0xf125e8]
	/usr/lib/libgconf-2.so.4(+0x16508) [0xeda508]