Revealing mistake: When Joey has the turkey on his head, and Chandler is by the door, look at the wall to Chandler's right. It's a "wild wall" - i.e., a movable wall put there to extend the set outward an additional few feet, since the camera normally doesn't see that far forward. The wall angles oddly away from the room, and you can see how the line and pattern of the hallway floor continues into the room.
Revealing mistake: When Chandler and Monica (and later Joey) are talking on the plane on the way back from London, the little display screen (showing a map of the Atlantic and coastlines) in front of Monica's seat shows the little grey plane indicating their position and direction. The plane is facing east, as though they are travelling towards London, but they are supposed to be flying back to New York.