Continuity mistake: Rachel is telling Ross why she doesn't want to go to the dinner, and she says "the first sign is I don't want to, and...I'm not going'. While she says this, her left hand is completely empty, but then in the next shot the camera goes back to Ross and she is now holding the baby monitor in that hand. It then moves around in different shots afterwards.
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: In the scene after Monica pretends to drop socks in the hallway in order to prevent Rachel from telling Ross she still loves him, Rachel goes back into the apartment and locks Monica out. Monica then starts to pound on the door with her bare left hand. The scene changes to the apartment and Ross opens the door to let Monica in. All of a sudden Monica is pounding the door with her right hand wrapped in a sock. (This continuity problem is for the widescreen version, where we see a closeup of Monica not seen on the DVD, in the hallway with a bare hand right before Ross opens the door. However, on the DVD there are extra shots of Ross and Rachel after seeing Monica's bare hand pounding and then seeing her with the sock on her hand, which gives her ample time to put it on).
Continuity mistake: When Joey says "we'll call that plan B, all right?", Monica has her chin in her hand. When it cuts to a different angle, she's got both arms on the table. Rachel also goes from having both hands clasped under her chin to having one arm resting on the table and one reaching for coffee.
Continuity mistake: In the scene where Ross takes a liquorice from the skull, in the first close up shot he is holding it in his hand, but then immediately after this when they zoom out, he is chewing a piece. This is impossible because he would have had no time to bite it.
Continuity mistake: When Phoebe gets the kids to sing along to 'That's another thing that you don't wanna do', there is a little boy and girl sitting perfectly still. The shot cuts to a close-up, and one of them is now standing and they are both swaying.
Continuity mistake: During the countdown to midnight, just as everyone says, "Happy New Year!", the camera cuts to Phoebe and Ross. In the background Joey and Rachel open their arms to hug each other; the camera cuts to just Joey and Rachel and they open their arms again.
Continuity mistake: When Monica and Chandler are in Las Vegas watching the burlesque show, Chandler turns the light on the middle of the table off to "hide" from his Dad. When Monica calls out saying "Waiter. Waitress.", the light is suddenly back on and remains on for the rest of the scene.
Continuity mistake: When Ross is telling Monica and Phoebe that he found his "sound" in college, Monica spits out her orange juice and places the glass on the coffee table. The shot changes to Phoebe and you can see Monica putting the glass down again.