Continuity mistake: When Chandler's in Rachel's office, she tells him the lingerie isn't kept there. He says, "Yes I realise that", and starts reaching for the catalogue she's about to give him. Camera cuts, and he's now standing with his hands in his pockets, reaching out again when she gives him the catalogue.
Continuity mistake: Chandler is able to get into Monica's apartment to tell everyone that Underdog got away, but later when Rachel closes the door it is now locked. We can see the door as each person is leaving and nobody does anything which could be construed as locking the door before it closes. It's been often demonstrated in the show that the door doesn't lock automatically.
Continuity mistake: When the friends are talking about the contest to prove who knows each other better, Monica says to Joey "Are you scared?" and Rachel has her arms crossed. Joey says "No" and in the next shot of Rachel she is crossing them again.
Continuity mistake: At the end when Chandler is singing "Endless Love", Phoebe comes in and when she walks over to Chandler he has his arms wrapped around the Lionel Richie record with the shoe in his hand in front of the record. When the shot switches to just Chandler, his right arm is resting on the arm of the chair and his left arm is holding the shoe behind the record.
Continuity mistake: Phoebe is complaining about people disrespecting her charity bucket, and hands the bucket to Monica as she does so. In the first two shots Monica holds the bucket by its edge, but when the camera goes back to her for a third time, she is holding the bucket by its handle.
Continuity mistake: There is a shot of Phoebe in Central Perk about to talk to Frank (from outside, looking in). From this shot there are several people outside Central Perk. The camera does an instant cut to behind Phoebe, (inside, looking outside) and the people have vanished.
Continuity mistake: Phoebe and Ross are in the soon to be closed bar sitting on the pool table. He clears it of everything, but a few seconds later he stops making out because the "damn balls are in the way" and somehow magics up a handful of snooker balls. If you look closely when he's clearing the table, there are two dark balls at the side which he studiously ignores, and which Lisa Kudrow then surreptitiously grabs and takes onto the table with her so David Schwimmer can grab them later.
Continuity mistake: Chandler walks into his apartment to find Rachel eating cheesecake without him. As the door is shutting, the pen on the Magnadoodle is hanging loose; it can be heard hitting the door when Chandler is closing it. The next shot shows Rachel stop eating, and when we next see Chandler, the door is closed and the pen is in its holder.
Continuity mistake: Ross is over at Chandler and Joey's looking through the peep hole for hours waiting for Rachel to return. When she does and Ross sees her go inside with Mark, Ross tries to open the door to go see her, and Chandler jumps on his back to stop him. As Chandler says "I...am not going to let you do this", Ross grabs the door handle. Then the angle changes, and as Chandler says "You...are surprisingly strong," Ross is further from the handle and grabs it again.
Continuity mistake: When Richard sits in Monica's room, when Monica opens the door we can see in the background the living room's window draperies, table, picture frame, etc., but they're not as they should be. In this mock-up the shade is pulled down, the draperies look different, and the direction they're all facing differs.