I've been playing for about 3 days and this problem has happened twice to me, the first time it happened after about 2 hours playing, the second time it happened, 10 minutes ago, i took a screenshot of it.
http://img25.imageshack.us/img25/6178/lolwati.jpg
i dont get this problem with any other game and the pixel errors persist when i shut the game down, I have to restart the computer to reset the picture.
my graphics card is a 9800GT 512MB
Comments
the other possibility is that your video card is overheating but not to the extent that it is crashing your computer. check that the graphics card is clean of dust, turn on v-sync in the game options to limit your card's framerates and thus hopefully make it run cooler, see if that helps. post back with results, it will help others and when the devs see your post, may help them. also you might post your computer specs so devs have further information to work with.
if you have not done so submit a bug report about this as well, it might help the devs, and certainly can not hurt.
thanks for confirming my suspicions.