Rrraou Roux

If this is what i think it is, I get this all the time at work. Basically what causes this in Max is when you model half your mesh, mirror and combine both halves to make the whole. It doesn't show in the Max viewport but the normals are inverted. The solution to this is, activate backface culling. Then when you attach the mirrored half, Reset xform the model. You'll probably see the normals flip. Apply a normals modifier, collapse all again. And it should be ok. Personally I like to spam the reset xform a few more times and recollapse to be sure. I've actually had models that didn't reset correctly on the first try. It's been like this for as long as I can remember.