Has this ever happened to you in MudBox, parts of the geometry (their UVs) are on different sides when doing a “Flatten to UV Space”?
The most common reason is that you have that polygon´s normals reversed, but if the normals of your geometry are correctly oriented, go to “UV Editor” and click on the “Toggle Shaded UV Display” button
There you have!, “Red UVs” or “Flipped”, a real headache if you have never seen before (I say from experience) and you dont know why you still give that failure in Mudbox despite your normals are perfectly (more than failure is a nuisance when it comes to texturizing)
This usually happens when modeling in symmetry, Having created an X-1 Instance (Y or Z, as the case) of our half geometry
But we easily fix them this way:
I hope this helps!!