I've got a strange issue, unless I plug in both monitors, I get frequent horizontal flicker artifacts, whenever there is movement on the screen (when I drag a window for example). With both monitors plugged in, and set to "extend desktop" the issue doesn't appear.
This is my office PC, no gaming, just regular work stuff.
Windows 7, 64-bit
AMD Radeon HD 6570
Latest drivers: Catalyst CC 2011.1109.2212.39826, 2D driver version 8.01.01.1215
The main monitor (which flickers when used alone) is connected via DVI, the other (extended) one via a DVI-VGA converter.