Comment 4 for bug 684992

I think you're right. It does seem to be some sort of video memory corruption. It doesn't happen immediately on boot but after about half an hour after I've run some applications. Happens to a different tooltip everytime, but once it happens that tooltip's stuck with the artefact until reboot. So I guess it does happen reliably

Strangely, it only happens to the tooltips (so far). Also, desktop effects in Maverick doesn't give any such artefact (that I noticed)