This slip up occurs in the episode where Monica, Chandler and Phoebe discover that they can eavesdrop on Ross & Charlie and Joey & Rachel through the walls of the hotel.

Joey and Rachel's room is on the left of the room Monica, Chandler and Phoebe are in. There are two doors in the room Monica & co. are in. One which leads to Joey's room and another which goes to the hallway.

Ross and Charlie are on the right side of Monica co.'s room. Somehow Ross goes through a door in his room and pokes his head through the door that is leads to the hallway in monica's room.