The One Where Ross Got High - S6-E9
Continuity mistake: When Rachel tells Phoebe that she's making a traditional English trifle, Monica's holding a cloth napkin, but in the next shot the napkin's suddenly gone and she's now holding forks.
Other mistake: In "TOW the cheap wedding dress" the whole episode is about how Chandler wants "The Swing Kings" to play at the wedding, because, at their concert, that is where he realised that Monica was the woman he wanted to dance all his dances with, and Monica goes to great lengths to secure the band. In this episode, the band at the wedding is actually called "Jungle Swing".
Suggested correction: So the band changed their name in the interim. It happens.
The One With Ross's Wedding, part 2 - S4-E24
Audio problem: When the wedding officiant says "take thee, Ross" his lips don't move. (00:20:35)
The One Where Ross Got High - S6-E9
Continuity mistake: When Phoebe finishes telling Rachel about her thanksgiving dream about Jack, Chandler is looking straight ahead with his arms crossed. Cut to the next shot when Jack and Judy are coming out of Monica and Chandler's room, and his arms are immediately uncrossed and he's looking at the women. (00:20:05)
The One Without The Ski Trip - S3-E17
Continuity mistake: At the start, just as Rachel is leaving Joey's apartment after mentioning the tickets she's got, Joey stops her and she turns round. Her hands are clasped together in front of her chest in one shot and by her side in the next. (00:04:00)
The One The Morning After - S3-E16
Continuity mistake: When Rachel asks, "When did Chloe leave?" the paper Ross is holding changes direction. Note the title of the paper: The New Yorker. (00:13:30)
The One With The Football - S3-E9
Continuity mistake: When they are outside for the first time, you see a shot from behind Monica, Joey, and Chandler. Monica then says, "Bring it in," and the shot changes. However, all 3 are now in different poses to what they were in before. (00:04:40)
The One With Phoebe's Ex-Partner - S3-E14
Continuity mistake: When Phoebe says you never run on a barge, her hand changes position on the table between consecutive shots. (00:06:50)
The One With All The Thanksgivings - S5-E8
Continuity mistake: The emergency room doctor's rubber gloves come and go between shots.
The One Where Ross And Rachel Take A Break - S3-E15
Visible crew/equipment: When Ross is on the phone to Rachel, near the start, just as he walks into the living area, a stage light is visible above him (only visible in fullscreen version). (00:06:10)
The One Where Monica and Richard Are Just Friends - S3-E13
Continuity mistake: When Monica is dabbing Richard's shirt, the way she is holding the cloth changes between shots. Note the amount over her fingers. (00:08:50)
The One With Ross's Inappropriate Song - S9-E7
Continuity mistake: When Chandler turns around and Joey watches the football game, the door to Chandler and Monica's room is open. However, when Joey sees the sex part of the tape in the next shot, as Chandler is about to turn around and the TV screen is visible, the door is shut. (00:14:29)
The One With Phoebe's Uterus - S4-E11
Revealing mistake: Phoebe already has a visible baby bump before she even agrees to get pregnant.
The One In Massapequa - S8-E18
Continuity mistake: When Monica shows Chandler the photo of Chi-Chi, as she says, "It's good stuff, huh," in the closeup Monica takes hold of the photo with her raised empty right hand, but in the next wide shot Monica is holding a Sharpie in her right hand and the photo is in her raised left hand. (00:02:25)
The One With The Stain - S8-E7
Continuity mistake: After Gunther responds to Ross with the biting "ezels" retort, it cuts to Monica and Brenda in the living room, and when Monica falls off her chair, the rug and blue pad underneath are suddenly rolled up at the corner, then the rug is properly flat. (00:11:15)
The One Where Ross Hugs Rachel - S6-E2
Other mistake: In this episode, Chandler says that "when it comes to sweets, [Joey] is surprisingly strict" and doesn't allow sweets around the house. Except that in Season Five, "The One With The Kips," we see that Joey leaves Rollo candies everywhere.
The One with the Monkey - S1-E10
Visible crew/equipment: When Joey the Elf deliberately jingles his elf shoes, Chandler's head may explode because of all the unspoken mocking jokes. It then cuts to a shot of Marcel, and the trainer's hand is visible in the bottom left corner of the screen. (00:07:25)
The One Where Rachel Tells... - S8-E3
Continuity mistake: After the opening credits, when Joey and Phoebe enter Joey's apartment, note the poster hanging on the wall and some items on top of the fridge, as well as a few of the magnets on the fridge, change, move, vanish, or suddenly appear in the following shots. Examples: the wall poster moves closer to the door; the Chunky B. Wear magnet at the side of the fridge moves to the freezer door; a takeout menu suddenly appears on the fridge door; the red star magnet vanishes; etc. (00:03:00)
The One With The Soap Opera Party - S9-E20
Other mistake: When Rachel walks in to her apartment right before the message of Jan Rogers, you can see the hole in the wall where the board usually is.
The One With The Halloween Party - S8-E6
Continuity mistake: When Joey is saying that he doesn't want to dress up, behind him is a guy with his shirt on top of his pants but in the next shot it is down. This happens throughout the whole scene. (00:00:35)