Comment 4 for bug 1313254

Found in bug #1264368 a possible explanation: gthumb may be passing bad/invalid paramters to `g_source_remove()`.

Also relevant:

https://bugzilla.gnome.org/show_bug.cgi?id=721369#c7

> It's not GTK+. GLib recently started throwing a warning when g_source_remove()
> is passed garbage (as per warning). Your applications have probably been broken
> for a while, and there's no telling what could actually have happened in the
> past when g_source_remove() would happily close any random source because the
> programmer got the wrong argument to g_source_remove().