Corrected entry: In the scene with Joey in the shower the camera cuts several times from Joey to the director. During this scene look to the background (behind the director) and you will see boxes which are marked with the name 'Monica'.
Corrected entry: At the beginning of the episode where they are all watching Joey's play, Chandler says that you could always spot someone who has never seen one of Joey's plays. At this time Phoebe is looking inside the leaflet about the play the shot changes to a shot of her and she is looking at the front of leaflet saying the title makes her jump.
Correction: Monica Geller is not the only Monica in Friends world. They could have been filled with things belonging to a character in the movie by that name.