The One with the Blackout - S1-E7
Audio problem: When Phoebe is counting down to the last candle burning out you can see Joey blowing out the candle, but the sound of his blow is only heard a second later after he sits back. (00:22:50)
The One With Ross's Grant - S10-E6
Continuity mistake: After Joey puts the video in the player, it's clear the video doesn't go all the way in, and even starts to eject itself. The shot cuts, and without him making any moves to replace the tape, it plays fine.
The One With The Princess Leia Fantasy - S3-E1
Continuity mistake: When Janice is talking to Joey about his disliking her, her long hair keeps changing position from hanging down her front to being behind her shoulders and down her back.
The One Where Ross Moves In - S5-E7
Revealing mistake: During Danny's party you can see Monica's apartment through the gap between the door and the wall, despite the party taking place on a lower floor. This is because they just reused the set for the hallway to save having to build a new set for the "lower floor."
The One With Joey's Porsche - S6-E5
Visible crew/equipment: When the camera follows Phoebe as she is going to clean up the vase she knocked over, the foot of the baby is still visible in the shot and you can see how someone picked it up as the foot goes straight up. Then as the camera moves back revealing there are only 2 babies, you can see a shadow of a crew member moving on the ground. (00:16:00)
The One With The Worst Best Man Ever - S4-E22
Continuity mistake: When Joey says that Ross needs to choose his best man before handing out of the ring, the camera cuts to Chandler. In the background you can see Joey getting up from the chair, but when it cuts back to Joey he is sitting down and then gets up in that shot. (00:02:01)
The One with the Birth - S1-E23
Character mistake: Outside the room in the corridor where Carol is waiting to deliver her baby, the poster outside giving directions says "Nusery", not "Nursery". (00:02:15)
The One Where Rachel Tells... - S8-E3
Continuity mistake: In a deleted scene, the security guard opens Monica's case. There is a blue item on top, when the camera changes, this item has disappeared.
The One With Rachel's Crush - S4-E13
Continuity mistake: When Monica waves the smell of cookies in the guys' new apartment, you see Ross right in front of Phoebe when they walk over, then you see Ross in front of Rachel as they go in.
The One With The Hypnosis Tape - S3-E18
Continuity mistake: When Pete is trying to ask Monica out at the diner she works at, she rejects him while placing the bell jar over the cheesecake and the pie server on the counter. In the next shot, the bell jar is nowhere to be seen and the pie server is up on the cake stand with less cheesecake. Then the bell jar reappears over the cheesecake. (00:07:00)
The One With The Jellyfish - S4-E1
Continuity mistake: When Monica gets stung by the jellyfish they are standing on a part of the beach that is wet and close to the surf but when she is saying "damn the jellyfish" the part they are standing on is dry and further from the water.
The One Where They All Turn Thirty - S7-E14
Continuity mistake: In The One Where They All Turn Thirty, it is Rachel's birthday. They show flashbacks of all the other Friends' thirtieth birthdays despite Monica's birthday supposedly being months after Rachel's. Plus, earlier on in the show Joey was 25 when Monica was 26, and since Monica and Rachel were in the same grade Joey should actually be a year younger than them, not a year older.
Continuity mistake: When Rachel and Monica knock on Danny's door and he opens it, there is a wall a few feet behind him. Later, when Rachel shows up at Danny's apartment to cancel their date, she enters into an open room and there is no longer a wall in front of the door. Danny's apartment is supposed to be one floor directly below Chandler and Joey's apartment. This was obviously the same set for their apartment that was used for Danny's and the wall was added for that shot to disguise it.
The One With The Inappropriate Sister - S5-E10
Continuity mistake: When Rachel and Monica are arguing about her meddling, Monica raises her hand to her face and you see a sparkly engagement ring on her finger. Her character wasn't engaged but Courteney Cox was at the time. (00:07:05)
The One With The Ultimate Fighting Champion - S3-E24
Continuity mistake: Joey's hair is suddenly much shorter after the intro with Robin Williams and Billy Crystal - everyone's wearing the same clothes so no time is meant to have passed.
The One Without The Ski Trip - S3-E17
Continuity mistake: When Phoebe is saying how she gets out of tickets, she is pointing at herself in one shot and not in the very next shot. (00:05:50)
The One With The Football - S3-E9
Continuity mistake: At the end, Chandler points at Joey, and when he does so, his sleeve goes from rolled down to rolled up instantly. (00:22:10)
The One With The Football - S3-E9
Visible crew/equipment: Just before Monica tackles Chandler to the floor, there is a shot with a very large boom mic visible at the top of the screen (only visible in fullscreen version). (00:13:10)
The One With The Football - S3-E9
Continuity mistake: Chandler says he's been preparing for something his whole life. When he says this, his right sleeve is rolled up. Camera changes, and now it's rolled down. (00:10:40)
The One With The Football - S3-E9
Continuity mistake: When Joey tells Chandler to prepare to feel very bad about himself, he has the football in his right hand. When the shot changes, it's in his left. (00:10:30)
