Continuity mistake: Right at the end when Ross hits Chandler with his rolled up magazine, he only folds over one-third of the magazine onto the middle third. When it cuts to him hitting Chandler (without a time delay between him hitting Chandler and folding the magazine) the magazine is fully folded over.
Continuity mistake: Just before Chandler gives Monica the mixed tape, he tries to manipulate a coat hanger into something feasible. He stretches it out and holds it up, saying "Yes, honey, I made it myself," then the camera cuts to another angle as he throws it down. After the cut the hanger is not as stretched as it was before.
Continuity mistake: Chandler is able to get into Monica's apartment to tell everyone that Underdog got away, but later when Rachel closes the door it is now locked. We can see the door as each person is leaving and nobody does anything which could be construed as locking the door before it closes. It's been often demonstrated in the show that the door doesn't lock automatically.
Continuity mistake: When Joey and Rachel are trying to work out if the other knows about Monica and Chandler, they sit at the table in silence. In a side shot of Joey they look at each other sneakily, and Joey has his arms by his sides. It then cuts to a head-on shot of Joey, and he's got his right arm over the back of the chair.
Continuity mistake: When Monica and Phoebe are looking at the dollhouse, Monica says "I don't want a ghost for the house." When she says this, the camera is behind Phoebe whose right hand is up in the air. The shot then changes to Phoebe from the front, and her hand has instantly gone to her lap.
Revealing mistake: When Phoebe is looking at the front page of the "Chelsea Reporter," Monica says her restaurant review is on the back page. However, Phoebe doesn't turn to the back page, but merely flips the paper over, looking at the bottom of the front page (and finding the review there).
Continuity mistake: In the episode with Chandler in a box, when Rachel is showing Ross all the things she had kept from their relationship in a shoe box, her hair is tucked behind her ear in most shots, but when she says "I keep the gifts that matter", it's all loose, then goes back to how it was in the following shot. Also in the shot before that line she's pointing the fossil at Ross, but when it cuts to a front view she's pointing it upwards.
Continuity mistake: In the one with Rachel's babyshower, Monica needs to ring Rachel's mom to invite her. She says that she has her number in her phone book and walks over to the phone - we see her start to look at it, but when the angle changes she's got the phone and is dialling.