I have a skinned mesh (character) which looks different in PCs and Macs. You can see that the helmet of the kid in this demo works fine in Mac, but looks like it has bad normals if you see it on a PC:
http://www.gustavom.com/movies/Unity/littleKid1.html
I will try to fix it by remodeling the helmet and the head, but I think it is an important issue to solve if it is not just bad user result.
Regards
I encountered the same problem. It seems, that it happens when the mesh isnt concav. I have an Asteroid of torus-shape in my space shooter, and the normals look swapped inside out, while all the other objects look as desired.
I have tested on two different pcs and on several macs. The issue always appeared on pc, but never on any mac. I got only Unity Indie, so i cannot tell, if it happens in a windows standalone as well.
Frank