[Brightcolours]

That is not exactly true. You cannot mount EF-S lenses on a full-frame EOS camera without modifications and risking damage of the mirror. Thus you should definitely consider EF and EF-S as different mounts.

Well, it kinda is true. Putting APS-C image circle lenses on FF does not make much sense, and the mount is the same with the same connections and the same flange distance and so on. 

There were rumors -- and I am not sure if these are confirmed now -- that you will not be able to mount Sony FE lenses onto the E-mount of APS-C (NEX) cameras. This would be the opposite of the EF/EF-S restriction.

That would be a bit odd, I can't think of a valid reason to do that... You never know with Sony, I guess?