Display wakes up unexpectedly without activity on Ubuntu 12.04

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http://askubuntu.com – When I use lock screen with ubuntu 12.04, all applications are hidden and a simple window asks me to enter my password to unlock the screen. After a while of inactivity, the screen goes black. However, only a few seconds after, it goes back to that window again as though the mouse had been moved, though that is not the case. Is this a known bug, and if so, how to get around it, and if not, then what I am doing wrong ? Thanks (HowTos)