Continuity mistake: After Chandler says "Merry Christmas" to the gang, they all respond - Phoebe just goes "blargh blargh", and has her hands in her lap. They all turn to look at her, and suddenly her hands are resting on the sides of the stool.
Visible crew/equipment: When Monica shuts the TV off while watching skiing, there is a closeup of the screen as it goes blank. You can see reflected in the blank screen an image of a man in a long sleeved shirt and tie, who does not correspond to any of the characters in the room.
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.
Continuity mistake: When Ross comes back to Monica and Rachel's apartment and Rachel tells him she has lost Marcel, he takes off his overcoat and lays it over the back of the sofa and then sits by it. As the scene continues, the sofa is still visible in some shots and the black coat is crumpled up on the sofa cushion.
Character mistake: In the early years, the gang are talking about their pets dying and their parents saying they sent them to a farm and Ross thinks that actually exists. He said that they sent their dog Chee Chee there and HE could run about chasing animals. Then in TOI Massapequa, Monica is preparing her speech and she tells Chandler about Chee Chee and said SHE died when Monica was in school, and when Parker asks whose dog it is, Monica says HE's my old dog. Then during her speech, Monica says HER and Nana are gone.
Continuity mistake: When Joey is talking to Ross and Chandler about missing Phoebe's birthday, Chandler has the newspaper in his hands. In the next shot, you can see that Chandler does not move at all, then in the next shot, the paper is on the table.
Other mistake: When Chandler says "We just got married. Right there!" and points. We hear a small 'ding' and Monica asks "Is that your ring?" But when Chandler pointed, you can see that all of his fingers except his index finger are closed to his hand. So the ring couldn't have come off.
Continuity mistake: When Phoebe and Monica are at Monica's table and Phoebe is trying to convince Monica to go on a date with her date's translator, watch Phoebe's arm. She points at Monica when she says, "Don't call him that; his name is Spackleback Larry", but when the shot changes to a side view of them, she has her arm resting on the table with no time to put it down between shots.
Continuity mistake: Ross and Joey are discussing Joey writing his own moviel; Ross sits down to begin writing, and Joey places a beer on the table next to him. The large part of the label is facing to the side, but it then switches to facing the front without being touched.