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by Alexey Loukianov on Tuesday September 21st 2010, 10:14
There was a huge bug in HL2 after the "HDR" update Valve had released in the middle of this year: when playing in non-HDR modes you would result in some levels appear overbright as the update lacks compiled lightmaps for some levels. This bug was reported on the steam forums ages ago but I don't know if there was an official reaction on this.
So to answer your question about weird effects we should first determine if this is a known bug in HL2 itself or a configuration/hardware-specific problem you had run into.