Hmmm there was an excellent (as always) post by highway61 here that now seems to have been deleted.
Anyway, the content of the post was that although both the Contax helical and the Nikon helical have the same thread pitch, a Nikon helical cannot be used on a Contax camera (or visa versa) just by shimming it. The reason is that the Nikon helical needs to rotate about 280 degrees to focus the lens from infinity down to 3 feet, and the Contax helical needs to rotate the about 290 degrees to focus the lens from infinity down to 3 feet.
It's this difference between the amount of helical rotation required for focus at a given distance that causes the incompatibility.
This causes a Contax lens to back focus on a Nikon camera, and a Nikon lens to front focus on a Contax camera. Depending on the aperture and focal length used, the DOF may or may not cover this inherent focus error.