Hmm. Well, I don't know what specific map you're talking about in CS:S since I don't play it, but I've seen the checker design on a map (Very sure it wasn't scout and knives) on my friend's PC. He was having Video Card problems.
I can't think of any reason for this other than the video card has something wrong with it's hardware.
You could wait until tomorrow and get some answers from the more experienced members.