What could be causing this screen problem? I have a Mac Book Pro Early 2011, with upgraded RAM (Mac manufactured) and i7 processor. It is currently connected through the miniDisplay Port to a second VGA monitor (using the miniDisplay Port to VGA adapter).

This morning I woke up the MBP that I had left under Sleep and was surprise to see this. The second monitor, however, did not show this problem although the mouse cursor was all pixelated, much like the MBP screen. 

Also, the computer was slow and unresponsive and I rebooted it.

Restarting my MBP fixed the problem.

It hasn't happened again so far, but it troubles me. What could have caused such a problem?
I would think that the graphics driver power management somehow failed. If you used a less sophisticated Operating System you would see this way more often. (Especially on Linux I saw patterns like that all the time the driver crashed or before the framebuffer got initialised). If your computer would have been manufactured earlier there was some problem with nvidia chips, however your computer should not be affected.