This can happen with quite a few widgets (though a crash is more likely, as you can see from the last faq). The example posted to the mailing list recently refers to GnomeDateEdit - it was being made available (using GladeXML's get_widget() call) as a gtk.HBox. What could it be?

When using Gnome and libglade, you MUST import gnome.ui before doing any GladeXML parsing. Failing to do that means gnome will never be initialized and libglade will just Do Bad Things<tm>