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.
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