S.T.A.L.K.E.R.: Clear Sky Message Board  

This sometimes happens to my 4870 GPUs (despite latest v1.5.05 patch), so all you have to do it restart the game when it happens after an hour anyway to make it disappear. Its worse the more aggressively you overclock your GPU so stick them on default clocks.

Here's the 2 screenshots of what happened to me:
Link
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I haven't gotten to Clear Sky yet, but the original STALKER had a similar problem. On a lot of systems when it rained there would be flashing white triangles. I didn't have that problem at first, but my graphic card died and I had to RMA it. The next two cards both had that problem and so I RMAed them, and then the next card did the same thing.

I tried all the fixes people suggested online before I finally found a solution. It was actually pretty simple, I downloaded RivaTuner and set my graphic card's fan to run at 80%. I started the game up and loaded a save I had made while it was raining and no more white triangles. I set the fan back to stock, loaded the same game save and the white triangles were back. I set the fan back to 80% and now just set the fan to 80% every time I upgrade my graphics card or install Windows.

I haven't seen the problem since.