Continuity mistake: When Phoebe says she is going to "pack her ass off" she walks toward the coffee house door, and they cut to a quick shot of Joey then back to the wide shot with the door, and Phoebe has disappeared. She did not have time to make it to the door and out. (Widescreen version).
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. (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).
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).
Visible crew/equipment: When Rachel tells her date she's going to have to take a rain check and goes back into the apartment, as she opens the door you can see a cameraman standing inside the apartment. (This is only visible in the broadcast version, not DVD or widescreen).