Revealing mistake: When Phoebe says something like, "Look, the last candle is about to go out", she starts counting down, saying "-47,-48..." and you can see Monica looking at the candle. But there is a light source behind her (it can't be a candle because she is looking at the last candle).
Continuity mistake: Joey goes into the bathroom and the toilet is behind the door. In a previous episode, when Joey doesn't know about Chandler and Monica, he comes out of his room and thinks it is morning. He goes into the bathroom and Chandler pushes the door open to reveal Joey asleep on the toilet with his toothbrush in his mouth, only the toilet is now on the other side of the room, not behind the door.
Continuity mistake: Paul finally opens up, and cries like a baby to Rachel. While talking to her at the Breakfast Bar (when Rachel is trying to distract him with sex), he's wearing a white linen shirt that constantly changes between tear-stained and wrinkled to ironed and clean.
Continuity mistake: In the one where Ross finds out he's having a boy (series 1), just before Rachel lets it slip, she is sitting on the edge of a chair eating a cookie. Watch her dress, it changes back and forth from being undone from the waist up, to only having two buttons open at the top.
Continuity mistake: When Monica comes in and asks Chandler if she can borrow some milk, Chandler is reading a paper that is flat on the bar. In the next shot he is holding it folded, then he lays it on the counter, and suddenly it is in his hands again.