Continuity mistake: When Phoebe says she is going to "pack her ass off" she walks toward the coffee house door, and they cut to another scene quickly and then back to the door and Phoebe has disappeared. She did not have time to make it to the door and out.
Continuity mistake: There is a scene in Central Perk where Rachel confronts Chandler about all the kissing he's been doing. She raises her hands up in front of her and tells Chandler to hold on to stop him from getting up from the couch. As he goes to sit back down on the couch she lowers her hands to her lap. The shot cuts to an angle behind Rachel's chair and she is lowering her hands again.
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.
Factual error: When Monica is in the bath with Chandler, Joey comes in twice, forcing Monica to hide under the bubbles: first he asks Chandler if he wants some chicken, then again when Chandler calls him back and Chandler's giving Monica's order, with Monica still hidden under the bubbles. Somehow, Monica supposedly hears Chandler requesting a soda and nudges or pulls him or something to indicate he was going to get the wrong kind. But no way she could hear him under the water (stick your head in a tub and have your friend recite the alphabet, and see what I mean).