In the comment box's editor's toolbar, if the mouse cursor is positioned at the top of a toolbar icon, then the tooltip continually flickers on and off. (I'm guessing that the tooltip shows where the mouse is, causing the mouse to no longer be hovering over the toolbar, causing the tooltip to be hidden, causing the mouse to be hovering over the toolbar, etc.)
Observed in Firefox 38.0.5 (where the tooltip flickers slowly) and Chrome 43 (where the tooltip flickers rapidly).