Other mistake: Right at the very end, when Joey is in the fat-suit and picks up a piece of food and says "How you doin'?" he doesn't put it in his mouth, he just lifts it to his lips, stops, and tries to stay still. If this had been a freeze on the part of the director then fair enough, but it isn't as he and particularly Monica are still moving. It just looks rather corny and amateur, as though they are waiting for the "cut" to be called.
Continuity mistake: Ross claims he looks into Monica and Chandler's and watches them eating dinner and watching TV. His apartment is a floor lower down than theirs and with the balcony wall, the sofa and tables distance from the window and the fact the bottom of the window is higher than the floor, there is no way Ross would be able to see that far in. We have seen in the past that he can basically only see who is at the window.
Visible crew/equipment: Just after Chandler says "I'm the smallest person in the world", and Joey comes out of his room, check the top left of the screen - you can see a boom mic track Matt LeBlanc across the set, and the bottom of some stage lights in the middle.
Revealing mistake: At the point in the ceremony where Joey forgets the rings, the camera pans out and you see Chandler step sidewards to get the ring from Ross, and Monica step sidewards to get the ring from Rachel and Phoebe. As Monica moves, her dress slides across and one of her feet is revealed; it has what looks like a brown boot or heavy brown shoe on it.
Continuity mistake: When Phoebe and Chandler are about to kiss there is a bottle of wine and two glasses on the counter. It then cuts to a view of Phoebe with no bottle or glasses on the counter, just a phone, and then later in the same scene it cuts back to Phoebe again and there are two glasses and a bottle of wine again.
Continuity mistake: During the scene where Phoebe is telling Rachel that Paolo made a pass at her, Rachel unrolls a long length of kitchen roll. During the scene there is a shot over her shoulder where we see that the kitchen roll is as normal and in the next shot we see it unrolled again.
Continuity mistake: When Monica returns from work to find that Chandler has been playing on Ms Pac Man all day, he lifts his hand and it is in a claw shape, which Monica comments about. However, when he and Monica are talking you can see the same hand and it's fine. He points to the screen of the game with it, and is also seen putting it into his pocket. When Phoebe enters later and asks why Chandler has suddenly gotten so good at the game, he shows her the same hand...the claw is back
Continuity mistake: The night before they go to the baby furniture store, Ross and Rachel are in the kitchen talking. Just before they leave the kitchen watch Rachel's robe: in one shot the 'lapels' are neatly one over the other, and in the next shot one lapel has moved lower and looks halfway folded over as if she hastily pulled the robe together.
Continuity mistake: When Monica and Phoebe are eating with the diplomat and his translator, watch the translator's chocolate cake. It either rotates, has some taken out of it, or is a different piece. Whichever way, the cake changes size, shape or position without him ever touching it.
Continuity mistake: When Monica is at the party and she is drunk, there is one point when she is sitting down on a chair with Chandler on her right. Chandler is holding her left hand (which has a snack in it), but when it cuts to a front view of Monica, suddenly both her hands are on her lap.
Plot hole: In the one with the red sweater they say that whoever it belongs to left it behind, but in the one where Rachel finds out that Ross videotaped them together it is clear that Ross is not wearing anything under the sweater, therefore for Ross to have left it behind he must have gone home topless.
Continuity mistake: In the one with Rachel's new dress, she's in her underwear on the sofa when Joshua comes in. In wide shots focusing on Joshua she's quite laid back, but in close shots of her she's sitting more upright. Worse than that, her legs are crossed, but uncross in the shot of his parents coming in, only to re-cross in the next shot when she gasps.