In the Christmas episode where Ross is talking to his son about Santa Claus, there is a slip-up. Both of them are sitting on the couch, and the camera keeps switching from Ross to his son. First his son's hair is down. Then the camera switches to Ross and back to his son, and his hair is now flipped up in the front. This goes on through the whole scene and is very noticeable. I think it happened because twins play the part of his son, so they must have switched, but they could at least have done their hair the same!