Continuity mistake: After Chandler has been playing the Ms Pacman machine all day he claims his hand has seized up and it appears to look like a claw. From then on in that scene, his hand manages to move around fine. It goes in his pocket, and he waves it around like normal, and then when he mentions his hand again at the end of the scene, it appears to have seized and is clawlike again.
Continuity mistake: Near the end when Rachel runs after Phoebe and Mike to ask Mike if the guy from the bar called, she leaves the apartment with her bag, which she places over her left shoulder. When she is talking to Mike the bag is gone, and then when she returns to the apartment she has her bag again.
Other mistake: When Joey comes out the bathroom in the girls' apartment with a turkey stuck on his head, the bathroom floor has the black and white tiles. But that floor wasn't there until Joey put it there in Series 3, The One With Frank Jr. So in the flashback the bathroom floor should be white tiles.
Continuity mistake: When Ross arrives at the classroom late and out of breath, he tosses his jacket at the hook, but it falls to the floor, and he walks behind the desk anyway. In the next wideshot of the class, as Ross still stands behind the desk, the coat is up on the hook. (Viewed on broadcast TV as a rerun, so there may be a missing shot that was removed for extra commercial time.).
Continuity mistake: When Chandler is telling Monica and Phoebe about the girl who picked him over Joey, look behind him and you will see Gunther nodding to a lady in a denim jacket. In the next shot, the woman walks up to the counter again, and again Gunther is nodding at her.
Other mistake: When Rachel leave her message on Ross' answering machine she mentioned the name Michael after 17 seconds and "I'm over you" after 25 seconds. When Ross listen to the tape he asks "Who is Michael ?" after 8 seconds. Rachel flips the phone out of his hand after 18 seconds so he couldn't have heard her "I'm over you."
Continuity mistake: Phoebe and her mother are in her mother's house at the beach. Phoebe has just found out that Phoebe senior is her real mother. Have a look at Phoebe's mother in this episode and the last episode of series 3 (The One at the Beach). Her hair is a lot shorter in series 4 even though no time has passed in the storyline.
Continuity mistake: In the last scene, Chandler is sitting on the couch in the new apartment, and the rooster is sitting on the back of the couch, with his tail facing the window. When Joey comes running out, the shot of the rooster shows his face toward the window, and then in the next shot he has returned to his tail facing the window.
Factual error: Monica takes an ovulation test to see if she is still ovulating. The ovulation test only works before you ovulate (usually 24 to 36 hrs), to detect the LH surge that comes before you ovulate. Once you ovulate that surge is gone and the test will not work.