The very bottom of the Ask Question button expands a few pixels down on hover after a short delay. Happens on Main and Meta, in Chrome latest, FF 9 and IE8. Looks worse in modern browsers as the rounding on the bottom corners alters slightly as well to fit the taller shape.
It doesn't look intentional (and if it was it doesn't look good regardless). The other similar buttons (like Post Your Answer) don't do it.