I don't know if this is the same thing, but I hopped into WCS for the first time last night. Playing as a lv2 undead with vampiric whatever, my screen flashed really brightly every time I leeched some health by shooting someone. I know that ATi cards have issues with fog that nVidia ones don't:
http://www.abload.de/image.php?%20img=screen2x3j.jpg
http://www.abload.de/image.php?%20img=screen1ccsln8.jpg

The explanation from the AMD/ATi forums is:
Its because newer cards dont have support for 16bit texture dithering so the textures are 16bit instead of a dithered 32bit. The game would need to be updated to 32bit textures and shaders to work correctly.
One poster suggested that turning Catalyst A.I. on in the ATi config panel resolved the problem for them. Might be worth checking to see if that option impacts this bug for you, Xerxes.