I've updated to the latest rawhide yesterday with this problem amongst others:

To reproduce:
-add a clock applet to your bottom panel
-set it to only display the time, not the date
-hold cursor over clock and wait

Expected results:
-tooltip with its bottom located above the panel / clock showing date

Actual result:
-tooltip with its bottom aligned with the bottom of the screen, covering the 
 mouse cursor, resulting in a fight to be on top between cursor and tooltip,
 which results in annoying flicker (and me not knowing the date).


I've also filed this upstream:
http://bugzilla.gnome.org/show_bug.cgi?id=457984