This error appeared after the important Upgrade.
A temporary solution is
- Go to tty1 with CTRL + ALT + F1 then get back to desktop with CTRL + ALT + F7
This is a bug in a driver that was included in the update. The bug has been fixed, and the fix will be released with elementary OS Loki. (I think the fix will also come to elementary OS Luna as part of a future update to the Ubuntu base.)
My temporary solution for this problem is as follows: If I know I will be away from the computer for a while, I press Ctrl + Alt + Del then logout and shut down. Then I restart when I get back. If I was working of the internet I use Restore session to get back to what I was doing.