Continuity mistake: When Rachel gets off the phone with Mark she sets the phone on the counter. In the next shot the phone is in the holder and it keeps going in and out.
Revealing mistake: At the end, when Rachel and Ross are talking on Monica and Rachel's balcony, the edge of the "brick" wall behind them has a gap in it. The facade is obviously a thin plastic sheet that is made to look like bricks and mortar, and it has pulled away somewhat from the surface it is attached to.
Continuity mistake: When Joey is picking up his bagel off the ground in Chandler's apartment he says something about the bagel hole being to small and he has it in his left hand. In the next shot it shows Chandler making a face and Joey's hand is empty. In the next shot Joey is sitting down and the bagel is back in his hand.
Continuity mistake: When Ross first tries to flirt with the pizza girl he has money in his hand. When Chandler walks over he gives his own money for the pizza, she leaves, and Ross still has his money. In the next shot when he shuts the door, his hands are empty, then it is back in the next shot when Ross yells at Chandler.
Continuity mistake: Toward the end of the episode, the exterior shot of Central Perk shows about 8-10 people inside the cafe, at the coffee bar, at the tables. But the next interior shot shows the cafe is empty except for our six Friends, lounging in the closed cafe while Rachel cleans up.
Continuity mistake: Rachel is telling Chandler to break off his relationship with Joanna, who had walked through the room and went into Chandler's bedroom, shutting the door. By the end of the conversation between Chandler and Rachel, the door is open but no one has entered or left the room.
Character mistake: Both lightning rounds at the end of the quiz are significantly longer than the 30 seconds that Ross claims they are. (Even though he is supposedly using a stopwatch to time them.) The guys' lightning round is actually 43 seconds long, and the girls' lightning round is 45 seconds long.
Continuity mistake: When Rachel and Phoebe are in Central Perk talking about the Princess Leia Fantasy, a guy in a yellow shirt and white pants is standing at the bar behind Phoebe. As they talk he walks to the door and leaves, but in the next shot he is standing behind Phoebe again, just as she says "That's the moment when she becomes a woman."
Continuity mistake: At the beginning when Rachel and Monica talk about the free coffee sample, Rachel puts it down on the counter to her right. In the next shot it's lying to her left with no time to have gotten there. After that it switches from the right to the left and back again.
Continuity mistake: When Joey brings his date home and Monica runs into the bedroom with Chandler, her foot dislodges a red pillow that is very close to a candle on the floor. As the scene goes on, the pillow alternates between being kicked out of place and being neatly arranged with the others.
Continuity mistake: In the scene where Phoebe tells the gang about her "Relaxitaxi" idea the position of Chandler's left arm changes between shots. In the frontal shots it's lying the back of the chair and in the shots from behind it's wrapped around Monica's shoulder.
Continuity mistake: In the beginning of the episode when Joey tells Chandler that he hasn't forgiven him for kissing one of his sisters, Monica and Chandler's TV, entertainment set and ferns are missing from the set and there are cables protruding from the wall. The area is completely bare except for the French poster. Everything's back to normal slightly later in the episode.
Other mistake: When Ross is at Monica's party getting Mr. Green's glasses and cigarettes he bumps into Mrs. Green and ends up having to come up with an alibi for the glasses and cigarettes. He then puts the glasses on, puts a cigarette in his mouth and says 'In fact I'm gonna go out into the hallway and fire up this bad boy'. At this point he walks past Rachel but he stops still as if frozen mid-step just before the shot cuts to him walking out into the hallway.