Jim, I've had this happen too. I gather you use Max and the HabWare Max2obj plugin. Load the geometry back into max and have a look at the invisible edges in those facets. I'll bet you'll find them rotated the wrong way. I used to think I just stank and modeling edges, but recently I've discovered that the Habware plugin rotates some invisible edges, sometimes creating folded facets. To fix it make the messed up edges visible and turn them the right way. If the edge is visible the pluging won't turn it.