I have been using 11.04 since it was released, I first did an upgrade but had this problem.
I ran a fresh install on another machine this problem eventualy reared its ugly head.
I do not know what triggers this so I am unable to replicate this error.
The only way to get it back is to reboot, logout or restart X all are very annoying to say the least.
I have attached screen shots to clarify.
I am using Ubuntu 11.04 64bit.
Many thanks in advance