At 18 min 10 sec into the film, when Peter is taking his glasses on and off in front of the beveled mirror. Off to the left in the bevels reflection you can see somone's hand moving AFTER Peter's hand. This is of the person holding the large eyeglasses that are being placed in front of the camera for the POV effect.
This actually was not a slip up at all. It's just the type of mirror that is hanging. there is a ridge in the design of the mirror so you are just seeing another reflection of Peter's hand and glasses...not someone else.