Specifically, we downcast a GIcon (which is an interface implemented by both GdkPixbuf and GThemedIcon) to a GdkPixbuf in several places in midori-view.c where it isn't always true that the icon actually is a GdkPixbuf—there's no pixbuf underlying GThemedIcons. This seems likely to be what's happening here; if so, I'd expect to see failed GDK_IS_PIXBUF assertions when this issue occurs.
Specifically, we downcast a GIcon (which is an interface implemented by both GdkPixbuf and GThemedIcon) to a GdkPixbuf in several places in midori-view.c where it isn't always true that the icon actually is a GdkPixbuf—there's no pixbuf underlying GThemedIcons. This seems likely to be what's happening here; if so, I'd expect to see failed GDK_IS_PIXBUF assertions when this issue occurs.