Continuity mistake: When Ross introduces himself to Mona, the reception place card that Mona's holding switches from her left hand to her right hand, between shots.
Revealing mistake: In the one after Monica and Chandler get married, Chandler says he can't dance because his shoes are slippery. If you look closely, he managed to slip so much by standing on the bottom of Monica's dress, not because of the shoes. (00:06:00)
Continuity mistake: When Monica tells Joey and Ross she is not pregnant, her hand goes to her chest, then down to her side, and then back to her chest. (00:02:10)
Continuity mistake: Just as Ross and Mona are about to dance, her arms are crossed. The camera cuts and they are on her hips. (00:16:05)
Continuity mistake: When Rachel spits out the champagne for the first time, her necklace is center chest. When the camera cuts it is far right. (00:13:25)
Continuity mistake: When Ross says , "Yes, I am." after Mona calls him sweet, His hands are by his side. Next shot one of them is by his chest. (00:16:30)
Continuity mistake: When Monica says she wants to dance, her hair is always changing depending on the shot. (00:21:30)
Continuity mistake: Near the end when Monica and Chandler are going on to dance for the last time, Monica speaks and her hair is neatly behind her right ear. The shot then looks to Chandler before going back to Monica immediately. However her hair is suddenly behind her left ear when she can't have moved it as her hands are in the shot at all times. (00:21:15)
Continuity mistake: When Joey and Chandler are discussing Joey's shoes, the length of the sleeve round Joey's neck changes depending on the shot. (00:11:20)
Continuity mistake: When Monica and Rachel are discussing Phoebe, Rachel's hands go from playing with the table to dry washing depending on the shot. (00:12:30)
Other mistake: When Chandler says "We just got married. Right there!" and points. We hear a small 'ding' and Monica asks "Is that your ring?" But when Chandler pointed, you can see that all of his fingers except his index finger are closed to his hand. So the ring couldn't have come off. (00:00:05)
Plot hole: Chandler and Ross are surprised to notice that Joey has very small feet. It's well beyond suspension of disbelief to think that at the least Chandler (who, apart from being his best friend, has been his roommate for 8 years) has never noticed the size of Joey's feet before. (00:11:20)
Other mistake: In "TOW the cheap wedding dress" the whole episode is about how Chandler wants "The Swing Kings" to play at the wedding, because, at their concert, that is where he realised that Monica was the woman he wanted to dance all his dances with, and Monica goes to great lengths to secure the band. In this episode, the band at the wedding is actually called "Jungle Swing".
Suggested correction: So the band changed their name in the interim. It happens.
Continuity mistake: Rachel and Monica are talking about Phoebe getting pregnant at the table. Rachel's hands change every time where the camera is showing them in a different angle.