Continuity mistake: At the very end when U2 are on the radio it shows Rachel picking up the phone to ring the radio station. Without the song skipping it cuts to Ross and straight away the DJ comes on and says that Rachel rang her and told her what Ross did. How did Rachel ring up within the space of a second or two and say what Ross did and get the message through on to the radio?
Continuity mistake: In this episode, Rachel tells Chandler that she thought he was gay until he spent Phoebe's entire birthday party talking to her breasts. But at this point since Rachel joined the group, Phoebe hasn't had a birthday yet; that occurs some episodes later, in "The One With Two Parts."
Character mistake: In the early years, the gang are talking about their pets dying and their parents saying they sent them to a farm and Ross thinks that actually exists. He said that they sent their dog Chee Chee there and HE could run about chasing animals. Then in TOI Massapequa, Monica is preparing her speech and she tells Chandler about Chee Chee and said SHE died when Monica was in college, and when Parker asks whose dog it is, Monica says HE's my old dog. Then during her speech, Monica says HER and Nana are gone.
Continuity mistake: During an earlier episode Monica gave up her dream wedding dress so Chandler could have The Swing Kings at their wedding. So it is a little surprising that the band they actually have are called Jungle Swing.
Continuity mistake: When we first see Ross at the museum, the woman he works with is holding a rock. In the first shot, she is holding onto it with her right hand at the bottom and just before the shot changes she turns away from Ross. In the next shot, she is facing Ross again and her right hand has moved on top of the rock.
Plot hole: In the beginning the Friends enter Central Perk and see that there are some other people on their couch. Monica looks fine and happy - even though later in the episode the other Friends say that she's been depressed for some days now, and she doesn't exit her department.
Continuity mistake: I know it's harsh, and I love the show, but gotta mention it. Series 7 begins right after the end of series 6, with the engagement celebrations continuing. However, in the few minutes that have elapsed, Chandler has somehow lost quite a bit of weight.
Continuity mistake: When the bullies are about to fight with Chandler and Ross, they decide to put all watches and keys into a hat. He takes the hat off and places it down and everyone puts the watches and keys in. The shot changes to a further out view and the bully has a different hat on his head while another hat is still placed where he put it. When there is a closer view of him, there is once again no hat on his head.
Continuity mistake: When Ross is telling Rachel and Phoebe about Charlie's past boyfriends, he refills two wine glasses. Then in the next shot, he puts his hands to his cheeks in shock because he just remembered something, without putting the wine bottle on the table. It just disappears from his hands.
Factual error: In the one where Joey and Chandler lose baby Ben on a transit city bus, they later have to pick him up at the transit city headquarters. The man working there assumes that one of them is the father. There is no way that a city official would just give away a baby without checking for ID or a birth certificate or proof of fathership of the child.