The One With The Giant Poking Device - S3-E8
Corrected entry: When Monica and Rachel are banging their heads against the wall, you can see someone in the background by the bathroom when Rachel walks away towards the living room in pain while saying, "if it isn't a headboard, it isn't worth it."
The One With The Giant Poking Device - S3-E8
Corrected entry: In this episode Monica and Rachel babysit for Ben, and Monica accidentally bumps his head on a beam/pole separating the kitchen and the living room. However, as far as I can recall, we never see this beam/pole prior to or after this particular episode. (I'm referring to the one in the middle of the room, not the one against the wall).
Correction: This pole is seen in many other episodes but most of the camera shots look into the living room so it's not easy to see. You can see it in some of the shots when people are coming into the apartment.
Correction: That is Phoebe talking on the phone.