Revealing mistake: The giant poking device is supposedly long enough to reach right across to the next building, but after they wake up Ugly Naked Guy, Joey says "retract the device!" and after pulling back about a foot of it the end suddenly pops up at the bottom of the screen.
Continuity mistake: Chandler is chasing Kathy down the street trying to get her attention. During a close-up of him running, just before the camera cuts away we see a guy's face on the left; when it cuts back to Chandler he runs for a while and then encounters the same guy (the dog-walker). Also, when he finally lands in front of Kathy, there is a bike rider coming towards them, but as the camera cuts back and forth between the two, the rider never passes them.
Continuity mistake: In the one where Rachel is in the hospital after having Emma she is in the bed feeding Emma and Ross is sitting right beside of her. They are talking about their relationship and what should come. If you will watch each shot as they go to showing Rachel's face then Ross's you can see that Rachel's hair in one shot is in front of her shoulder and the next shot it is behind her shoulder. This happens several times.
Visible crew/equipment: When Monica, Ross, Chandler and Rachel are at the apartment and Rachel is suggesting how funny Chandler is, as soon as Ross says, "Sweetie, calm down it's going to be OK," a boom mic can be seen for a short period of time, at the top of the screen.
Other mistake: In the scene where Rachel is talking to Chandler in the hall, after she says "the 4th of July", it cuts to a shot from behind Rachel's head. If you look on the right hand side of the screen, you can see a piece of wood in the foreground. This probably came from Joey and Chandler's doorframe, through which this view would have been filmed. In addition, this door was supposed to have been closed throughout the entire duration of this scene.
Continuity mistake: When Monica comes in and asks Chandler if she can borrow some milk, Chandler is reading a paper that is flat on the bar. In the next shot he is holding it folded, then he lays it on the counter, and suddenly it is in his hands again.
Continuity mistake: When Phoebe, Chandler, Monica, and Joey are eating the organic wax, Monica has her feet up on the wall with her legs crossed left over right, but in Joey's closeups when he asks about needing a new walk, Monica's legs are crossed right over left.
Continuity mistake: While Rachel and Ross are fighting, when Rachel realises that she was at Ross' apartment at the same time as his "little friend", Ross holds out the newspaper (so she can hit him) with his left hand, but in the next shot it's held out in his right hand.
Continuity mistake: While Monica's preparing her shake in the blender she asks Rachel about the anniversary dinner, and when Rachel tells her that she and Ross broke up, Monica throws the blender lid and it flies over the counter, but in the next shot the lid is back in Monica's hand, and then it vanishes again.