The One Where Ross Dates a Student - S6-E18
Continuity mistake: When Rachel and Joey are at Joey's apartment throwing wet paper towels at the entertainment unit, Phoebe comes in to ask Rachel to switch back to Monica's apartment. During the scene, Phoebe is standing in front of the entertainment unit, and the wet paper towels' number and positions change between shots. (00:18:10)
The One Where Ross Dates a Student - S6-E18
Visible crew/equipment: While Phoebe and Rachel are with Monica in the guestroom, Phoebe decides that Rachel should be the one to get the nice room, and right before Monica tells Rachel that she's the first guest at "Hotel Monica" we can see the end of the bedroom set wall, with the kitchen and living room visible.
The One Where Ross Dates a Student - S6-E18
Other mistake: Ross' semester supposedly just ended, and he's handing in his grades. But the episode just before this one was about Valentine's Day, and the one immediately after this one is about spring break, so this episode is happening sometime between mid - February and March/April. It doesn't matter when the show was aired or produced; Valentine's day is still in February. There's no semester that ends at that time of the year, and NYU wasn't on the quarter system then.
Suggested correction: This episode (production code 225567) was produced before episode 17, "The One With Unagi", (production code 225568) and just aired out of order. So this episode happens before Valentine's Day.
The One With Joey's Fridge - S6-E19
Continuity mistake: The handle to the fridge door is on the left, so when it opens the inside is visible. In earlier episodes the handle was on the right and it opened out towards the room, blocking the inside from view. (00:00:05)
The One With Joey's Fridge - S6-E19
Continuity mistake: When Joey pushes Ross against the fridge, a box falls from the top of the fridge onto the shelf beside it. A few seconds later when Joey turns to Chandler, we can see over his shoulder that the box is back on the fridge. (00:08:55)
The One With Joey's Fridge - S6-E19
Revealing mistake: In the closing scene where Ross and Elizabeth are dancing on the beach, you can see that the sky in the background is actually a picture, and you can see where it ends. (00:21:20)
The One With Joey's Fridge - S6-E19
Visible crew/equipment: After Ross nervously sputters Professor Friesen's name as "Fee-sen-stin-linder" when he walks in on Ross with Elizabeth, it then cuts to Ross, Chandler, and Joey walking into Joey's apartment, and we can see the opening in the wall where the bulletin board usually hangs.
The One With Joey's Fridge - S6-E19
Visible crew/equipment: In the scene where Rachel introduces Sebatian to Chandler, Monica and Phoebe, there is a visible boom mic at the top of the screen in the wide shot, right after Chandler says, "Turns out he is kinda funny".
The One With Mac & C.H.E.E.S.E. - S6-E20
Plot hole: When Chandler is in Joey's apartment, he answers the phone and says, "Joey's not here right now, can I take a message?" A split-second later, we hear Chandler say, "Yeah, so the audition's been moved from 5:00 to 2:30?" It would be 100% impossible, within the fraction of a second that Chandler is silent, for the other party to pass along the fact that Joey's audition has been moved and the new start time. In fact, there wouldn't have been time to speak more than a couple of syllables. (00:08:45)
The One With Mac & C.H.E.E.S.E. - S6-E20
Continuity mistake: When Chandler answers Joey's phone there is a drawing on the magnadoodle. However in the next shot it has vanished with no time. (00:17:20)
The One With Mac & C.H.E.E.S.E. - S6-E20
Continuity mistake: After Chandler has written the message on the Magna Doodle, the sliding eraser moves from the right to left side just as he leaves with Monica, and when Joey and Chandler are playing foosball, the barely legible "Joey" is now neatly written on the Magna Doodle, and its sliding eraser is again back to the right side.
The One With Mac & C.H.E.E.S.E. - S6-E20
Continuity mistake: In the episode before, Rachel's hair is just underneath her shoulders. Then, a few weeks later when Ross gets back from Spring Break, it's a lot longer than before.
The One With Mac & C.H.E.E.S.E. - S6-E20
Other mistake: When we see the chess board at Central Perk that Ross and Chandler are using, the black pawn in front of the black knight nearest the camera is missing, and from the arrangement of the white pieces, it cannot have been captured yet.
The One With Mac & C.H.E.E.S.E. - S6-E20
Continuity mistake: As Chandler is writing on the board to remind himself about Joey's audition, the door opens as he's writing "Joey", and there are a bunch of squiggly lines under the y. As Chandler is thinking back at that moment and it moves to present tense, he looks over to the board, and the squiggly lines are different.
The One With Mac & C.H.E.E.S.E. - S6-E20
Visible crew/equipment: A stage light can be seen at the top left hand side of the screen just after Joey slams the door shut after practising a scene with Phoebe. It is visible in front of the blue picture on the wall.
The One Where Ross Meets Elizabeth's Dad - S6-E21
Continuity mistake: When Chandler is about to put the toilet paper on the table, the picture with the little girl is facing Phoebe. In the next shot, the picture is facing Chandler. Two shots later, it's facing Phoebe again, and then Chandler again in the next shot. (00:08:15)
The One Where Ross Meets Elizabeth's Dad - S6-E21
Continuity mistake: When Monica and Chandler enter the apartment arguing about what time they were supposed to meet, Monica has her handbag strap over her right shoulder, and then takes it off. When the shot switches to Phoebe claiming to know what time they said they would meet, the handbag strap is back over Monica's right shoulder, and then she takes it off again. (00:16:40)
The One Where Paul's The Man - S6-E22
Continuity mistake: When Rachel and Paul are on the couch at the cabin, the diet coke Rachel is holding switches (repeatedly throughout the scene) from her left hand to her right. (00:12:00)
The One Where Paul's The Man - S6-E22
Continuity mistake: When Monica is listening to the answering machine, the bread in the blue and white bag behind her changes position. (00:12:40)
The One Where Paul's The Man - S6-E22
Continuity mistake: When we hear the recording of Chandler answering the phone, it sounds nothing like what Chandler actually said. (00:13:00)