The bug is well documented in the apple forums with a possible workaround:
http://discussions.apple.com/message.jspa?messageID=11376362#11376362
In a few words my Mac mini's keyboard will stop responding randomly when switching spaces and the spaces overlay stays on the screen but I can still change spaces with the keyboard shortcuts and put it into sleep mode. To workaround it I have to put the Mac to sleep mode and then wake it up again. Does anyone know of an actual solution to the problem instead of the workaround?