We've been playing the same world since 1.1, and I recently noticed that none of the players can interact with the mobs they had previously tamed (cats, dogs, parrots).

Examining the save and comparing it with previous backups, the UniqueID of the players has changed, but all the animals still have the old OwnerID, so they now belong to nobody.

This has happened with the same world in realms, and also opening an older copy of the world in Minecraft 1.4.2 on Windows 10 (last used on 1.3.x, before moving it to realms). I'm no sure when it started happening, I just noticed it yesterday on 1.4.2.