This problem is visible with most programs that use GtkNotebook and stick a
button in the tab's label (using a GtkHBox layout element or whatever). 
For example, Gaim, Epiphany and gEdit all use some variation of this.

Replication of this bug is fairly straightforward.  Start up one of the
above programs and open a new tab (however this is accomplished in the
particular program).  The previously selected tab's "close button" should
now have a relief/bevel/whatever.

It does not keep this state, however.  When the button recieves a mouseover
event, it gets reverted to the assigned behavior and (in at least the
programs mentioned above) flattens itself.

Switching the "corrected" tab into active context and then inactive again
produces the bug another time, and so on.