A lot of times I need to modify the geometry of the faces that are mated. Sometimes I go from a planar face to a non-planar face, other times the other way around.
The main problem is that 2 planar faces accept only coincident mates between them and not tangent mates. I suggest that tangent mates should work also; it would make my customer's lives so much easier.
I posted it here, because currently I cannot seem to be able to submit ERs via the customer portal. There is no field of serial numbers in this image: