When Chandler is trying Ross's "hug and roll" trick with Janice, Chandler's arm gets stuck under her. As he's thinking of a way to pull his arm out, Janice raises her eyebrows and smiles, though she's supposedly asleep.
David Arquette plays Malcolm, a guy stalking Ursula. In one scene, he's talking to Phoebe at the subway, but his lips aren't moving.
When Chandler comes in to talk to Ross and Rachel, to discuss him accidentally calling Janice fat, the shadow of a person walking can be seen under the green cupboard door and the back of Monica's apartment by the balcony.
As Ross starts telling Chandler about the 'hug and roll' technique, a flickering blue screen from one of the cameras is visible reflected in the apartment window above the sink.
When Ross and Rachel are in the empty apartment together, as the camera is panning across the room, stage lighting can be seen above the bathroom door.
When Phoebe enters Monica's apartment and is questioned about 'the maniac', just as Phoebe kneels down and says "he's yummy, we did a little kissing", a boom mic is visible at the top of the screen just above Rachael's head.