I first noticed something wrong when using MR's physical sun & sky (which is what i would like to eventually render with). The shadows looked good in the beginning of the scene, but then some weird artifacts started to occur (see attachment).
So I tried rendering with a single directional light with ray-traced shadows, and was still getting incorrect shadows. The best example I came up with was to render a frame with only one of my objects visible, and then again with all objects visible (second attachment). When I did this, I saw the proof that something was wrong, because shadows that used to exist, being cast from just one object, suddenly disappeared after adding more objects!
I've played with all the quality settings I could find related to shadows in mental ray, but i'm really no rendering expert, and have no idea how to fix this. Anyone have any ideas?