Continuity mistake: When Phoebe and Ross are standing in front of the screen looking at all the babies and talking about why He and Rachel aren't together, some of Phoebe's hair is in front of her shoulder on the right hand side then the shot changes looking over Phoebe's shoulder at Ross. Then when you see Phoebe again her hair is behind her shoulder yet she has not moved or touched her hair.
Continuity mistake: When Ross comes into Chandler and Joey's apartment to ask the guys and Rachel what they are doing in four weeks, the Magna Doodle on the door behind him is blank. However, when he tells them in the same scene that Emily always wanted to get married where her parents did, the Magna Doodle says 'get out'. No character had any chance to change it. (DVD).
Continuity mistake: Just after Chandler explains the situation to Janice, she goes off crying, and Ross takes a photo. The flash is much brighter than it should be - the screen's whited out to disguise a jump cut - after the flash Chandler's noticeably further back than he was before.
Other mistake: When Joey and Chandler are in the cab on the bridge, Joey quickly stops the cab so Chandler can get out. However, on the screen behind, the background doesn't 'slow down' as Joey does, it just continues as if the cab is going at the same speed. Then, in the next shot, it is still. When Joey sets off again, the background stays the same and doesn't show that the cab is getting faster.
Continuity mistake: Rachel and Phoebe are in the Central Perk, talking about leaving the group, and a jacket and a handbag are hanging on the arm of the chair behind Rachel, slightly to her left. Between shots it changes from just the jacket, then just the handbag and so on. This continues throughout the scene when Ross comes in and Phoebe leaves. As she gets up you see both again, however Rachel does not move enough to just show one and hide the other.