I saw this one day, while I was working away using my Mac. (The menu at the top was not for the application that I was running!)
My Mac was just not a happy camper.
So yes, a reboot cleared its little head of all the woes it was pondering.
Another idea came to me from Macworld. If you are a Mac owner, go to Library > Caches. Select the entire contents of that folder and trash it. Restart.
Before doing this, I'd quit every program that is running. I have done it several times without any problems. However... Warning: Don't try this suggestion at home. (In other words, don't blame me if you have any resulting problems!) I'd recommend that you try it after doing a complete backup with a bootable external disk. For the technically-inclined, that means using a tool like SuperDuper with a Firewire drive.