Continuity mistake: When the six are looking out the window at Ugly Naked Guy cooking his turkey, they are looking straight across the alley into his window. In "TOW The Giant Poking Device", Ugly Naked Guy's apartment is several floors below theirs in the adjoining building.
Continuity mistake: In the shot where Joey meets Kristen the lights in the store window behind them are switched on. On the DVD at 05:19, you can see that the "CD" light is illuminated in blue. However, at 05:36 where they both shake hands, all the lights are switched off.
Visible crew/equipment: In the scene where Rachel is trying to hide Ross from Paul, when he's about to go into Paul's bedroom after he says "Did you really hear a bear?", look at the window behind Rachel when the shot is on her. You can see a large light reflected in the glass and it moves slightly (not just in conjunction with the camera, it moves before the camera shifts).
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.