This is a bug, but fixing it means rewriting some of the photon reflection code, which is just hard to do. At certain angles, the PHOT just doesn't know where to reflect, and ends up not moving and killing itself because of that. (It's not that it's too expensive to calculate, just that it doesn't know how). changing the reflection code might break some things also anyway. I think jacksonmj was working on redoing it a little a while ago, I can't remember how far he got on that.

You could just adjust the angle very slightly, that always fixes it