Comment 55 for bug 931967

Unity checks for "bad" PCI-IDs and graphics drivers every time ubuntu 12.x boots up. Based on the decision if your gpu is "bad", it will then automatically present either the Unity 3d or the 2d environment.

I think this graphical corruption the user sees is the hidden checking process that runs in the background and tests the gpu's features or polls for the device-id and the driver being in use.