Continuity mistake: When Rachel is talking with Phoebe about Paolo,. she unrolls some kitchen paper and uses it as a tissue. Initially she's holding two torn halves attached to the roll, then when saying "He's a pig" it goes to a whole sheet not attached to any others, then when she sits down it's a whole sheet attached to the roll again. Plus when Phoebe sits too, in some shots the paper is on the table, in others it's not.
Continuity mistake: During the scene where Phoebe is telling Rachel that Paolo made a pass at her, Rachel unrolls a long length of kitchen roll. During the scene there is a shot over her shoulder where we see that the kitchen roll is as normal and in the next shot we see it unrolled again.
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: During the song at the beginning of the episode, keep your eye on Ross's mug. He is holding it in his lap, then there is a shot of Monica with Ross's empty saucer on the table. In the next shot, the mug has appeared on the saucer.