In the last few days I've had a sudden flash of gray in the background. I'm on MacOS 10.7.5. I'm not able to provide a screenshot because it only happens for a split second, but here is a more or less accurate mock-up:
It happens on average 1 time per hour, though it's not regular. Sometimes 5 mins pass till the next flash of gray. It covers multi-monitors and it also happens if my external monitor is not connected.