Yep, any time you mirror geometry (use symmetry instead it doesn't screw up the UVs) or use Flip in the UV Editor window (use rotate as best you can) it turns the UV pieces inside out. MR looks at the back of the poly, sees through it and renders nothing. The quick way to fix it is to go into the UV Editor window > Face…
Okay, this is the second time I have run into this problem and I have no clue what could be causing it. The only work around I have found is to not use mental ray all together. I read another post a while ago where EricChadwick and Vig were talking and thought the solution was there but I guess I was wrong because it…