Mickey lived on a parallel world for about two years, and after he returns to his original world, he probably keeps discovering ways in which the earths diverged. Idioms that he finally learned how to say on the other earth he has to relearn how to say the way he first learned to. History wasn’t his best subject to begin with, but sometimes he starts talking to Martha about countries that either don’t exist anymore or never did in this timeline. There are sports teams and models of cars and brands of cereal and all sorts of things where he can remember the names but he can’t always remember which world they were from. He’ll get a jingle stuck in his head and none of his friends can help him remember what it was for because it never aired in their world. He’ll learn of a celebrity death and be certain that person already died- but wait, that was in the Cybermen attack, that must have been the other world. Just because he’s home now doesn’t mean his time on the other world is forgotten, and sometimes he feels so isolated from the world around him because there are all these things he thinks should be one way that are now another.