The One With The Fertility Test - S9-E21
Continuity mistake: Just before we see Janice, Monica has her hands on Chandlers chest. When it cuts to the wide shot, her hands are round his neck. When Chandler says 'Aw come on.', her hands are on his chest again. (00:12:50)
The One With The Fertility Test - S9-E21
Continuity mistake: When Charlie tells Ross about the Met, Ross goes sitting next to her, and his hands are empty when he says: 'Joey really knows his art, huh?' When Charlie says: 'Not so much' Ross suddenly holds a book in his hands. Then, when the shot changes, the book is gone again. It keeps (dis)appearing during the scene. (00:20:35)
The One With The Donor - S9-E22
Continuity mistake: When Monica is asking Zack if he or any of his blood relatives has diabetes, Zack's cutlery changes position twice. In the shot facing him he has his fork in his left hand and his knife in his right. When it changes to a side shot, he is gesturing and doesn't have anything in his hands, without having time to put the cutlery down. Then when it cuts back to a front shot, he picks up his fork, which is now on his right, and starts eating. (00:14:30)
The One With The Donor - S9-E22
Continuity mistake: When Phoebe enters the coffee house to get Joey's advice on her outfit for Mike's sister's party, it cuts to Joey saying "You look stop-eating hot", and Phoebe's left hand can be seen making a nervous up-down motion in response. But when it cuts back to her, it is her right hand making the motion and her purse is now in her left hand.
The One With The Donor - S9-E22
Continuity mistake: In the first scene in the coffee shop Emma is in a carriage beside Rachel. Later on when they come back to the coffee shop Rachel leaves and the baby carriage and the baby are nowhere to be seen.
The One In Barbados, part 1 - S9-E23
Continuity mistake: When Monica and Phoebe are talking about David, the ornaments in Monica's glass change between shots. (00:11:30)
The One In Barbados, part 2 - S9-E24
Continuity mistake: As Mike and Monica are playing ping-ping, two people playing checkers magically materialize behind Monica. One second they're not there and the next second they are. (00:12:25)
The One In Barbados, part 2 - S9-E24
Continuity mistake: In the final episode of series 9 part 2 where Phoebe is talking to first David and then Mike the shot keeps changing. Each time it does the amount of hair down Phoebe's back changes from a little to a lot. (00:23:30)
The One In Barbados, part 2 - S9-E24
Continuity mistake: In the scene where David wants to propose, when Phoebe and David get seated in the restaurant Phoebe puts her bag on the floor to the left of her chair. When there is a wider shot again, it has vanished. (00:25:25)
The One In Barbados, part 2 - S9-E24
Continuity mistake: When Monica and Mike play table tennis and Monica says "I think that may have missed the table" there is no one sitting in the chairs behind her. The shot then cuts to Mike and he replies "oh do you?" The shot then cuts back to Monica, she replies "er, yeah" and there are suddenly two people sitting in the chairs behind her. Then in the next shot of Chandler and Phoebe the women who were just in the chairs are now walking behind Chandler and Phoebe in the direction of the chairs, and in the next shot they take their seats. (00:38:40)
The One In Barbados, part 2 - S9-E24
Continuity mistake: In the One in Barbados (part 2), just after Monica bashes her hand against the table, the shot of Chandler asking if she's OK shows Monica raising a finger right at the edge of the screen. We then cut to Monica, who's cradling both her hands together. (00:42:50)
The One In Barbados, part 2 - S9-E24
Continuity mistake: At the end of the ping pong scene when Chandler wins, a paddle appears on the floor for one shot then disappears in the next. No-one had time to touch it. (00:44:20)
The One In Barbados, part 2 - S9-E24
Continuity mistake: In The One In Barbados, part 2, when Ross and Charlie are hiding behind the plant from the other scientists, the drinks they are holding refill themselves and Charlie's ornament in her drink keeps switching sides of the glass. (00:45:10)
The One In Barbados, part 2 - S9-E24
Continuity mistake: At the end of the episode, when Joey walks into Rachel's hotel room, he is standing in the doorway. However, when Joey and Rachel kiss, there is enough space for the door to close. (00:47:05)
The One After Joey And Rachel Kiss - S10-E1
Continuity mistake: When Ross and Charlie are sitting on the bed it shows Charlie fixing her T-shirt. In the very next shot her hands are on her lap. (00:04:25)
The One After Joey And Rachel Kiss - S10-E1
Continuity mistake: Chandler, Monica and Phoebe are shown listening at the wall between their room and the room Ross is in with Charlie. However, when Ross comes into the room, the outside hallway is shown, and it is clear that the wall they were listening at actually connects to the hallway, not another room. Ross even walks in that direction when he leaves, proving it's not a dead end or similar. (00:05:30)

The One After Joey And Rachel Kiss - S10-E1
Continuity mistake: At the end, Ross walks into Joey's apartment, he walks past the door. Next shot he is level with the middle of the door. (00:27:00)
The One After Joey And Rachel Kiss - S10-E1
Continuity mistake: At the end of this episode, Ross enters the door of Joey's apartment and finds Rachel and Joey kissing. Rachel says "I'm so sorry". The next episode, The One When Ross Is Fine, shows the same scene but it has been changed: Rachel never says "I'm so sorry". Joey starts talking when Ross enters the room.
The One Where Ross Is Fine - S10-E2
Continuity mistake: When Owen is telling his parents Chandler paid him $50 not to tell them he knows he is adopted, Chandler says "which technically now you should give back" and he is looking at Owen. In the next shot his back is to Owen.

The One With Ross's Tan - S10-E3
Continuity mistake: At the start, the lid on Joey's bottle is on and off depending on the shot. (00:00:20)
Answer: The statements that she's contradicting (that it is common, it happens to every guy, and it's not a big deal), are the things that a woman commonly says to a man who is suffering from erectile difficulty, typically to assuage his bruised ego. However, most men do not believe that these statements are true, as evidenced by Chandler's outburst. He's so caught up in the proof that women are lying about it that he gives himself away.
Rooster of Doom