The One With The Giant Poking Device - S3-E8
Continuity mistake: At the start, Phoebe snaps at Rachel that she will go to the dentist. When she says this, her grip on the apple in her hand changes between shots. (00:02:30)
The One With Ross's Inappropriate Song - S9-E7
Audio problem: When Emma laughs right before Ross comes in, the laughs don't match with her lips.
Continuity mistake: When Joey and Chandler talk about how to tell Monica that it is too soon for marriage, Joey fills his plate up with scrambled eggs. When he walks over and sits at the table, the eggs appear smaller on his plate. (00:15:54)
The One With Chandler In A Box - S4-E8
Visible crew/equipment: After Joey tells Chandler he's going to his folks and might stay there for a few days while looking for an apartment, just as Joey retrieves his keys from the shelf, a crew member wearing a black shirt and white pants is visible on the left side of the screen. (00:08:40)
The One Where No One's Ready - S3-E2
Continuity mistake: Just before Rachel and Ross are leaving Monica's apartment at the end, Rachel stops by the door but the next shot they're further in the room, then they walk past the door and out.
The One Without The Ski Trip - S3-E17
Continuity mistake: When Monica and Phoebe enter the coffee house and talk to Chandler and Ross, the extra sitting at the coffee bar (black shirt, blue jeans) is not wearing a belt, then when they pan again she is, and back she is not. (00:00:12 - 00:00:54)
The One With The Ride-Along - S5-E20
Plot hole: When Rachel picks up the box of photos they all fall out, because the box isn't sealed with tape or even folded properly. But there's no way for that to have happened. Monica would have brought the box from somewhere and put it on the couch, and the photos would have fallen out then, and she would have fixed it (this is Monica, after all.) The only way for this to have happened is if she put the empty box on the couch, unfolded/unsealed, and then put photos in it - which Monica would not do.
The One with the East German Laundry Detergent - S1-E5
Continuity mistake: The positions of the mugs shift between shots.
The One With Chandler's Dad - S7-E22
Continuity mistake: When Ross is in the car with Rachel he sat in the back you can see him he has his arms crossed and then when the camera cuts to the front view he is holding the seat in front. (00:04:50)
The One With Joey's Big Break - S5-E22
Other mistake: When Ross is questioning Phoebe on why she is mad at him, he has his blue sweater on backwards.
Suggested correction: So he has his sweater on backwards. That's a character mistake, not a movie mistake.
The One With The Halloween Party - S8-E6
Other mistake: When Monica is saying "I can't answer that, Chandler's my husband" after Joey asks her who would win in a fight between Ross and Chandler, the camera is fluttering in and out of focus. (00:11:11)
The One With Joey's New Brain - S7-E15
Visible crew/equipment: When Monica and Chandler are going over to the window to see Ross playing the bagpipes, the head of a crew member is visible on the left side of the screen, standing by the framed poster. (00:07:30)
The One Where Monica Gets a Roommate - S1-E1
Other mistake: At the end when Ross and Rachel split an Oreo, you can see that Rachel is holding 2 parts and Ross somehow still has one. (00:20:38)
Character mistake: In some episodes the facade of Central Perk is shown. On the window the word "cappuccino" is misspelled, it says "cappucino."
The One with the East German Laundry Detergent - S1-E5
Continuity mistake: When the horrible woman says "no suds, no saves", Rachel's hands suddenly appear on her basket. (00:11:30)
The One with the Candy Hearts - S1-E14
Visible crew/equipment: Just as Joey and his date are leaving, a stage light and the top of the set drop into view above Chandler. (00:08:10)
The One with the East German Laundry Detergent - S1-E5
Continuity mistake: Phoebe says she's going to break up with Tony, and Monica has her cheque book in her hand pointing away from her lap. Camera cuts and now it's in her lap. (00:03:00)
The One with the Dozen Lasagnas - S1-E12
Audio problem: The very last line of the episode (Monica saying "and I'm gone") is an overlay. (00:22:40)
The One with the East German Laundry Detergent - S1-E5
Continuity mistake: At the start, Chandler and Joey have their hands outstretched. Camera cuts and now Chandler's hand is lowered. (00:00:50)
The One with George Stephanopoulos - S1-E4
Continuity mistake: Monica asks Phoebe if she has a plan to which she replies she doesn't even have a "Pla." During this, her hairs position on the futon changes between shots. (00:14:40)