In version 1.0.2553, when "Post Effects" is turned on (even if all individual effects are turned off), and the resolution is set to 1920x1080 (all others are fine).. I was getting a 3-way split overlay from one of the shaders.
I haven't been able to get a screenshot, one fix is to modify /Users/*/Library/Preferences/Frictional Games/Penumbra/Overture/settings.cfg and set the resolution to 1920x1200 instead of 1920x1080.
This should help those with 24" monitors, since 1920x1080 is only native to HDTV's.
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It would be nice if this were an option, since it would fix this issue and prevent screen stretching.
Edit: Note that other graphic glitches, like cut-off Inventory screens and missing text during the first dark rooms is still an issue :/
Mac Pro 3.0 Ghz
Nvidia 8800GT 512MB
4 GB RAM
OS X 10.5.2 + Leopard Graphics Update 1.0