Continuity mistake: In the one with Phoebe's rat babies, it is very obvious that the box has nothing in it. Also when she is in her apartment explaining about giving up the triplets she had for her brother she puts the box down on the table, yet when it cuts to Mike's face and Phoebe's realisation that she hadn't told him that before, the box is back on her lap.
Continuity mistake: When Monica goes to Rachel's office and invites Gavin to Rachel's birthday party Rachel gets up and goes to the door exclaiming "why did you invite him, I hate that guy." She has nothing in her hand when she gets up from her desk; but at the very end of the scene when she tells Monica to be quiet, she's got a pen in her hand for one shot.
Continuity mistake: As Rachel says "the guy hates me", in a wide shot Monica turns to her with her arms by her sides to say "does he?". It then cuts to a shot more focused on Monica where she laughs, and she's now reaching forwards to the table.
Continuity mistake: In the series nine episode with the 'hot' nanny, at the end when Rachel is on the veranda/balcony thing with Gavin, before he says 'I do have feelings for you', the shot is over the shoulder and he has his hand on the wall of the balcony, when the shot switches to the front angle his hand is by his side.