/3/ - 3DCG

Usually when I make models I bake ambient occlusion to a texture and then use that as a basis for painting the texture, for example the positions of the thumbsticks on this controller should show up as black spots on the texture. (pls excuse shitty modelling)
However occasionally when I bake the ao one or more of the faces of the model will appear to be flipped, as seen in pic. It stays like this even if I rebake it, and I haven't done anything fucky with the uv map to cause a face to be flipped like that.
Any idea why this happens? I can flip the faces afterwards to match up with the ao map but the texture being split like that makes painting the textures annoying.