If you're baking from High to low poly model you'll need to go into face selection in the low poly model and hide the faces (not the same as hiding the object). -Edit- too slow!
Hey. Bumping this thread because it's happening again, and the answers in this thread aren't fixing it this time. the faces in the low face sub obj are hidden, no uv faces are overlapping or inverted, all the settings to my knowledge are correct....so im at a loss here. cage is good, high geometry is selceted, mental ray…
More then likely the UV pieces need to be flipped. If you're using 3dsmax go into the UVEditor window, select face mode, click Select > Select Inverted Faces. Flip anything it highlights. If you used the mirror tool it inverts your UV's and they always need to be flipped.