Hello, I just recently purchased a Sapphire Radeon HD 4850.
I installed it, and installed the latest drivers on the website, CCC 9.12.
I then wanted to play some games on some high settings.
I played L4D2 and then I noticed that there was some green flash when I was playing. It's hard to describe the green flash. It's not a green screen, just a part of the screen, and it wasn't just a box of it. It took the shape of the surroundings it covered up. It's like the spray paint can of things.
Use HWMonitor or GPU-Z to keep an eye on temperatures. Is this a new build, or are you replacing another card? If you are, what was it and did you remove its drivers and then run driver sweeper? Also, what is the card's clockspeed (is it OC'ed or Factory OC'ed)?
-Is the card new? Call/email the vendor and ask them. Its probably worth returning.
-2nd hand? Maybe the card was sold because it was faulty.
Check the card on a different machine and try replicate the failure. If you can, its an issue with the card, simple.
As for why AA causes an error...
1.) AA puts more load on the card. When cards die they can die slowly. Extra load often causes errors not seen otherwise.
2.) The AA 'chip/section/woteva' of the card has become faulty while all other components are working fine.
I played Warcraft 3 TFT today, and it pops up in that game, in the places I can't see, the green pops up in the 'fog of war'. WC3 doesn't have an anti-aliasing option, so I don't know how I can turn that off.
If I can replicate this on my brothers new computer he's building, then it is probably the card that is faulty.
If not, then I will email Sapphire about this issue.
NOTE: I tried taking screen shots of this, but it is not possible. When WC3 game ends, you can save a replay. It happens also on that, but not in the same areas. Maybe it is my monitor? I'm not sure.