The One Where Chandler Can't Cry - S6-E14
Continuity mistake: Jill goes to Ross' place, intending to make Rachel mad. When Ross goes to Rachel's to talk to her afterward, she rolls up her sleeves saying, "I'll give you something to think about." When Rachel is next shown, her sleeves are rolled down. Her arms also go from by her sides to crossed in front of her. (00:19:10)
The One With The Ring - S6-E23
Plot hole: When Phoebe accidentally allows Chandler's ring to be sold, she tells him that she overheard which restaurant the guy said he was taking his girlfriend to, so they can go find him and persuade him to exchange rings. However, in the previous scene we see that Phoebe was paying no attention at all to that guy - she was too busy trying on jewellery. She wasn't even within earshot of the guy, and we see from her reaction that she hadn't even noticed him at all before he left the store with the ring (during which time he doesn't say anything at all). (00:16:50)
Suggested correction: She may have been trying on all the jewellery, but she would have certainly heard the guy say he was going to a certain restaurant with his girlfriend before he left. Obviously she then realised the ring the man bought was the one Chandler had his eye on so reacted.
The One With The Apothecary Table - S6-E11
Continuity mistake: In the scene where Monica and Janine are in the hall, Janine goes to run and lifts up her arms, but the next shot shows her lifting up her arms again. (00:19:50)
The One Where Phoebe Runs - S6-E7
Continuity mistake: When Phoebe is waiting for Rachel behind the tree a man wearing grey runs past (left to right). Then when Phoebe jumps out and is talking to Rachel about her fake sore leg, the man in grey runs past again. (00:08:00)
The One Where Phoebe Runs - S6-E7
Continuity mistake: In the episode 'TOW Phoebe Runs' when Rachel runs into the horse at the end there is a shot of the horse looking round at her, when the shot changes angle to show Rachel picking herself up you can see that the horse is still looking straight ahead. (00:18:35)
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)
Continuity mistake: During breakfast, when Joey and Phoebe are talking about driving back, the fork in Phoebe's left hand disappears and reappears. (00:03:55)
The One With The Proposal, part 2 - S6-E25
Continuity mistake: At the end when Chandler returns to his apartment, from the outside he opens the door with his right hand. Then it cuts to an inside view and he's opening it with his left hand on the handle. (00:17:40)
The One Where Ross Dates a Student - S6-E18
Continuity mistake: When Monica is standing in front of Phoebe, asking if the smell from Joey's is bothering her, the shots of Monica from the front show her arms held out in front of her, but when the shot changes her arms are crossed. This happens twice. (00:11:15)
The One With The Apothecary Table - S6-E11
Continuity mistake: When Janine goes into the hallway to apologize to Monica about her behavior at dinner, she runs into Monica who says, "What did I tell you about the hall?" As she says this, she is pointing her right forefinger at Janine and her right shoulder is leaned in closer to her. When Janine says "I was just coming over here to apologize for my behavior," the camera cuts to Monica, who is now standing with her left shoulder leaning in toward her. The camera cuts back to Janine as she says "I'd really like it if we could be friends" and then back to Monica for her next line, where she again has switched to her right shoulder leaning toward Janine. (00:19:20)

The One With Joey's Porsche - S6-E5
Continuity mistake: Phoebe knocks over a vase and has to clean it up because "Let's face it; we're at Monica's." When she turns back around and notices one of the babies is missing, the baby on the right (in the striped top) falls over towards the other baby. A split second later when the shot changes the baby is sitting upright again. (00:15:15)
The One Where Paul's The Man - S6-E22
Other mistake: There are two mistakes in the scene where Phoebe spots Joey's picture in the laundromat. For one thing, when Phoebe says "Oh, here it is", she is looking in the opposite direction of the picture. Also, when she says this, her mouth doesn't move at all. (00:16:10)
The One With The Routine - S6-E10
Continuity mistake: When the Snippy Guy is pointing out couples to go on the platforms next, he points to one couple with his left hand, while holding a clipboard in his right hand. In the very next shot he points to the next couple, but this time with his right hand while holding the clipboard in his left. (00:11:15)
The One With The Apothecary Table - S6-E11
Continuity mistake: In the very first scene when Janine and Joey are kissing at the table in Central Perk, the blue cup by Joey's arm moves forward a few inches without him touching it. (00:01:25)
The One With Ross's Teeth - S6-E8
Revealing mistake: When Ross's date turns the lights down he can speak and yet the whiteness of his teeth doesn't show. (00:17:25)

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 Hugs Rachel - S6-E2
Continuity mistake: In the scene where Rachel is all happy that the three of them are going to live together, her arms keep changing position throughout the scene. (00:08:35)
The One With The Proposal, part 1 - S6-E24
Continuity mistake: When they are in Central Perk deciding who will go with Rachel to the auction, Rachel's purse and the cups in front of Joey move around. (00:02:20)

The One With The Apothecary Table - S6-E11
Visible crew/equipment: After Rachel tells Ross that Phoebe hates Pottery Barn, when she tells him that Phoebe will be there any second and they need to cover the apothecary table, a crewmember can be seen at the left side of the screen.
The One With Joey's Porsche - S6-E5
Factual error: Phoebe discovers that one of the babies has wandered away, and finds the baby inside the open drawer. Yeah, no. These babies aren't even walking yet, much less are they able to lift themselves up past the height of their own bodies and launch themselves into a drawer.