Factual error: Ross falls asleep on the train and ends up in Montreal, where the woman he meets tells him "it's only a 2-hour ferry ride to Nova Scotia." Yeah, more like a journey of between 13 hours (12-hour car ride plus 1-hour ferry) and 23 hours (multiple bus legs plus 1-hour ferry.) The character could have been lying to Ross, but it's not presented that way at all.
Character mistake: After the guys have beaten the girls in the quiz, Rachel refuses to move apartments and Monica says: "If we had lost, we would have made them get rid of the birds." But they did lose. That's why the guys have got their apartment. She should have said if we had won.
Other mistake: When Joey comes out the bathroom in the girls' apartment with a turkey stuck on his head, the bathroom floor has the black and white tiles. But that floor wasn't there until Joey put it there in Series 3, The One With Frank Jr. So in the flashback the bathroom floor should be white tiles. (00:04:45)
Revealing mistake: When Chandler grabs his bag he accidentally lets the box full of wedding china fall off the stool. Watch closely (play in slow-motion). The box falls on the floor and shattered china falls out. The china was ALREADY broken before it falls on the ground. (00:23:00)
Revealing mistake: After Phoebe tells Rachel that Paolo made a pass at her, we see a shot of the outside of Monica's apartment block and Rachel is throwing Paolo's clothes out of the window. Look closely and you can see the clothes have been very badly superimposed on to the background. (00:17:05)
Audio problem: When Joey walks out of the bedroom dressed as a 19 year old, when Chandler says the line "Yes, on a scale from 1 to 10, 10 being the dumbest a person can look, you are definitely 19", you can see that the audio doesn't match the words. It looks as though Matthew Perry says "not 19" and they had to re-dub over post production with "19." Subtle, but it's there. (00:08:50)
Continuity mistake: Rachel is telling Ross why she doesn't want to go to the dinner, and she says "the first sign is I don't want to, and...I'm not going'. While she says this, her left hand is completely empty, but then in the next shot the camera goes back to Ross and she is now holding the baby monitor in that hand. It then moves around in different shots afterwards. (00:06:10)
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)
Audio problem: When Ross and Joey go into the bedroom, you can hear the duck in the background, even though the chick and the duck should be in the other apartment and aren't even in this episode. (00:12:53)
Visible crew/equipment: During the song Phoebe made about her mother's suicide, when she says "How was I supposed to know my mom was dead in the kitchen?", a crew member is reflected on her guitar. (00:04:10)