Revealing mistake: Just before we see everyone setting the wedding up outside, there is a sweep shot of the city with snow falling. It's very obvious that this snow is added digitally and it's very badly done. There is no snow on any of the buildings, and there is no snow hitting the surface of the water. (00:20:50)David Mercier
Revealing mistake: At the wedding, none of the snow melts when it has been sitting on people for several minutes. Occasionally when it gets brushed off clothes it just sticks to hands and sleeves, again for several minutes without melting. (00:21:15)David Mercier
Continuity mistake: In the scene where Joey and Rachel are sitting on the couch in their apartment after they decide their relationship is hopeless Rachel puts her head on Joey's shoulder. Her hair is on her face but when she lifts her head up in the very next shot her hair is now behind her right ear which she couldn't have done because her hands never left her lap. (00:22:00)
Continuity mistake: When Monica goes to Rachel's office and invites Gavin to Rachel's birthday party Rachel gets up and goes to the door exclaiming "why did you invite him, I hate that guy." She has nothing in her hand when she gets up from her desk; but at the very end of the scene when she tells Monica to be quiet, she's got a pen in her hand for one shot. (00:12:45)
Continuity mistake: When the girls and Mr Goodbody are talking about paying him the three hundred dollars, the top buttons of his shirt are ripped open. Half way though the scene this changes to only one button being open and his shirt looking neat and pressed, as if he had not started his strip yet. (00:13:25)panda
Other mistake: In his apartment from later episodes, Ross' kitchen door is on the same wall as the front door, and it is a long and narrow kitchen, but Ross' apartment is between two others. The floor plan doesn't make sense unless the kitchen is only a foot wide, otherwise it would jut off into the hallway.
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)
Continuity mistake: As Rachel says "the guy hates me", in a wide shot Monica turns to her with her arms by her sides to say "does he?". It then cuts to a shot more focused on Monica where she laughs, and she's now reaching forwards to the table. (00:13:10)Jon Sandys
Continuity mistake: In the series nine episode with the 'hot' nanny, at the end when Rachel is on the veranda/balcony thing with Gavin, before he says 'I do have feelings for you', the shot is over the shoulder and he has his hand on the wall of the balcony, when the shot switches to the front angle his hand is by his side. (00:20:55)
Factual error: The guys are gone for a single night, watching the Knicks game. (You can tell this because they talk about how "we can go see the game tonight," and when they get back everyone's wearing the same clothes as that conversation.) When they get back, both apartments are completely switched back: the large, heavy furniture is replaced, the posters are re-hung, etc. Even if they were gone from, say, 6 p.m. to midnight or so, there is no way that Rachel and Monica would be able to make that transition in that kind of time.