http://forums.steampowered.com/forums/showthread.php?t=915039
Either there is an issue with the program (above), or your memory would not allow that block of memory to be written at that time. Either thats okay (it just got confused) or bad, there is an issue with your memory's timing/voltage/physical issue.
Try replicate the error, if it doesnt happen then dw. If it does happen try the above etc. Or possibly re-install cs.