Continuity mistake: When Joey and Chandler are at Joey's colleague's brunch, you can see a big wall clock in some shots. You can see it when Chandler says: "Wow, there's my fantasy's come true" with the minute hand at five to the hour. Just a few seconds later, when Joey says: "Can you see me in a place like this?" it can be seen when Chandler is in shot, and the minute hand is now at 20 to the hour.
Continuity mistake: When they're all sitting together and having pizza, talking about parents, Rachel has no napkin. When she says: "Just think, in a couple of years we get to turn into them", a bunched up napkin appears beside her on the couch. After Chandler's remark it has disappeared again, then it appears and reappears again between shots.
Continuity mistake: Just after Rachel has tried to whistle and just blew out crumbs, she picks up the fork with her left hand and places it to the left of the cake on the kitchen counter. In the next shot just before Chandler picks up the cake the fork is back on the right side of the cake near her right hand.
Continuity mistake: When Ross and Monica are at the wedding reception talking to the other guests at their table, a lady wearing a red scarf is standing behind them with her back turned. Throughout the scene the red scarf keeps changing position very quickly - in one shot it's slid down one shoulder, then back up in the next and then slid down again. At the end of the scene it's off her back entirely and she has just slung it over her arms.
Continuity mistake: Between season 4 and season 5 some of the furniture in Chandler's and Joey's hotel room is changing. In TOW Ross' Wedding the three big pictures over the beds show dark spiky plants like ferns and palm trees (as can be seen in the scene where Joey shows Chandler the video with Fergie). In this episode these pictures are completely different - they show a red flower, a sunflower and a dark green plant. The two pictures next to the door are now much smaller in bigger frames and the lamp on the night table between the beds which had a white shade in season 4, now has a black shade. The nightstand between the two beds also has a different shape.
Continuity mistake: When Rachel returns from her first interview at Ralph Lauren she takes out a bottle of water out of the fridge. In the following shot her hands are empty and there are only Chandler's and Joey's bottles on the table. Rachel's bottle reappears on the table next to Chandler's and Joey's bottles (that are also standing closer together than before) for a moment just when Chandler starts saying "Well, you could have tried not kissing him" and then vanishes again.
Continuity mistake: Joey is watching Ross from Monica's window and goes searching for the hot girl. The exterior shot crossing the street between buildings is inconsistent with the interior shots, which show them looking at each other across an alley with no visible fire escapes. The exterior shot crosses a street to a building with street-facing fire escapes on every apartment.
Continuity mistake: In the scene where Ross and Carol are talking and Carol says that all he needs is to find a woman who likes men, a woman comes across the room walking in high heels. The shot is wrong because we see her up until she's half the way through the table, and the next shot shows her four steps back.