While playing around with the client side windows branch of GTK+, I hit an assertion (specific to that GTK+ branch) with any page containing flash.
Alexander Larsson tracked this down to a GdkWindow being moved/resized during an double-buffered expose event:
While the assertion is now replaced by "fixing" the situation, it should be investigated why this happens and if it's possible to avoid it. Window operations during expose events sounds like a bad thing to do anyway.
*** This bug has been marked as a duplicate of bug 72805 ***