External display flickers after upgrade to OS X Mountain Lion

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Symptoms


After upgrade to OS X Mountain Lion a Virtual Machine in Fullscreen mode flickers frequently. This issue occurs only if a second display connected to your Mac computer

Cause

This OS X issue occurs on ATI 6xxx video card  on OS X Mountain Lion only. 

Solution

Download and install the latest OS X 10.8.1 update: click on Apple logo > Software update... and install the update

If the issue still persists

Disconnect the second display from Mac

Workaround 2

Disable 3D acceleration in your Virtual Machine:

Workaround 3

Use Fullscreen mode on one display only

Workaround 4

Use Coherence mode instead of Fullscreen one

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