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ATI and corrupted graphics
From Quake Wiki
Do you have corrupted graphics running Quake on your ATI graphics card like this?
Fix it by adding -bpp 32 to your commandline. Quake will then start in 32 bit color depth. It will use slightly more video memory but nowadays that is negligible.
Source: http://www.celephais.net/board/view_thread.php?id=60306&start=113&end=121