Continuity mistake: In the opening scene in Central Perk when Paul (Elizabeth's dad) takes Ross to one side to talk to him you can see the girl in the blue top enters the door twice: first the shot after Paul says "I'm funny like that" and then again after Ross says "I realise it upsets you".
Continuity mistake: When Chandler kisses Monica before letting the social worker in, she puts her hand on his arm. The camera cuts to her and she says "Tongue, really?" but her hand is by her side now. The camera cuts back to Chandler and both her hands are now out in front of her.
Other mistake: Mid episode when Monica turns off the cordless phone after being told by Wendy that she can not make it, the phone makes the "out of range" audio tone instead of the "power off" audio tone. Occurs directly as Monica says, "Wendy, we had a deal, yeah, you promised. Wendy, Wendy, Wendy.".
Continuity mistake: In the scene where Phoebe asks Mike to prove that he plays piano, his arm goes from resting on the couch to on his lap from one shot to the next. After Phoebe tells him "You are really good," his left arm is resting on the couch. But in the next shot when Phoebe continues talking, his arm is resting on his lap.
Continuity mistake: When Jack and Judy leave Joey's apartment, there is a birthday banner on the counter that has fallen off. When Monica, Chandler, Joey and Phoebe are racing the toys to see who gets to go, the banner is back on. In this time no-one has fixed the banner as there is not enough time for them to do so off camera and the people on camera don't fix it. It's most clear that it's back on when Monica is saying, "Technically, our dog never crossed the finish line".