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)
Other mistake: Megan tells Monica that she booked the Swing Kings the day of Monica and Chandler's wedding and that she can only have them back if Monica gives her the cheap wedding dress that they both want. But in reality, a professional wedding band would never book a night for a client without receiving a deposit first. It just isn't feasible that Megan would put down a deposit for a band she had no intention of actually using (her wedding wasn't even that same day), especially given that she was looking to cut costs, hence shopping for a cheaper designer dress. There's also the fact that by the end of the episode Monica's booked the Swing Kings but is still in possession of the wedding dress - she could tell Megan to get bent and just keep the dress and the band!
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.
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)
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)
Other mistake: When Chandler finds out that the video marked 'Monica' is just a football game he says, 'This may be the first time I have enjoyed football.' Although in 'TOW The Football' (season 3) he is quite happily watching the Thanksgiving Day game with Ross and Joey.domwhit23
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)
Revealing mistake: When Ross comes back from asking out his neighbour at the counter, he has his hand open at one point, pointing down, but then apparently smashes the egg on Chandler. It's clear he never had the egg in his hand as there is no mess on Chandler's vest top either.