If I start emacs with GTK+ accessibility support enabled, I get the
following warnings:

 ** Message: ATK_ROLE_TOOLTIP object found, but doesn't look like a tooltip.
 ** Message: ATK_ROLE_TOOLTIP object found, but doesn't look like a tooltip.
 ** Message: ATK_ROLE_TOOLTIP object found, but doesn't look like a tooltip.

To reproduce, set GTK_MODULES=gail:atk-bridge, start emacs from a
terminal, make sure x-gtk-use-system-tooltips is t, place the mouse
pointer on a toolbar button, and wait a second.

After quick search I found a similar issue in Pidgin (solved):
http://developer.pidgin.im/ticket/2723
I'm attaching a patch based on this.