When I was mousing over the "Top Contributor" icon in the homepage's right panel (see image below), the tooltip starting flickering in and out.
It only happened when my cursor was hovering over both the user's image icon and the tooltip. My browser's style inspector also showed the "in" class getting added/deleted rapidly.
This effect only happened in Firefox. Chrome/Safari didn't have the flicker but the tooltip was aligned a bit far left / down over the icon.
Love the site guys! Good luck!