I just noticed this in troubleshooting performance in Diablo III.

If you have your mode set to Fullscreen (does not seem to do this with windowed or fullscreen windowed) and you ALT+TAB out and back into the game it will sometimes cause framerate issues. What I noticed happened is that it caused my nvidia control panel setting of "VSYNC Forced On" to stop working and I started to get screen tearing.

To fix I had to restart the game and all was back to normal again.