top tool bar turns black sometimes...why?

Every once in a while the toolbar at the top of my screen on my macbook turns completely black and I can barely make out what is hidden underneath. it covers everything from the little apple all the way to the time and date across the screen. Why does this happen and how do I get it to go away?

this happened to me yesterday, i just shut down my mac and it cleared the problem, dont know why it happened thought !!!

Same here

This happens to me sometimes, too. I haven't been able to figure out why it happens, but I know that restarting my computer usually fixes the problem.