Comment 18 for bug 662503

This is definitely a Compiz bug, as it also affects Ubuntu, and not just Linux Mint. It's very apparent in Linux Mint though because by default the clock is on a bottom panel. However, if you move the clock to the bottom panel in Ubuntu as well, the same problem occurs.

I was planning on installing Linux Mint 10 64-bit, so that's what I'm going to do right now and then try out the fix.