Continuity mistake: When Monica is asking Zack if he or any of his blood relatives has diabetes, Zack's cutlery changes position twice. In the shot facing him he has his fork in his left hand and his knife in his right. When it changes to a side shot, he is gesturing and doesn't have anything in his hands, without having time to put the cutlery down. Then when it cuts back to a front shot, he picks up his fork, which is now on his right, and starts eating.
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.