Continuity mistake: When Joey is counting the floors of Ross' building (the first time) to find out where Hot Girl lives, he is wearing a black top with a zip. When he rushes over to Ross' building he walks down the hall and knocks on what he believes to be Hot Girl's door. In these shots he is wearing a dark purple button up shirt. As soon as Ross answers the door, Joey is wearing the black top with a zip again.
Continuity mistake: When Phoebe is in the coffee house talking to her father and pretending to be the executor of her grandmother's will, after she says "lipstick and a daughter, big day for you" she folds her papers in half, but in the next shot they are unfolded and she then folds them again.
Continuity mistake: In the flashback where Joey sticks the turkey on his head, the subtitle says Thanksgiving 1992. However, the show started in 1994 and Joey didn't move in until the previous fall, so the earliest Thanksgiving that he was there would have been 1993.
Continuity mistake: Monica and Chandler are piling their dinner plates with food while arranging to meet for a second quickie. Chandler's plate is stacked with various items when he agrees to meet Monica in the cellar, but when he tosses his plate back onto the table to rush off without eating anything, the plate is nearly empty.
Continuity mistake: In the scene where Joey has just brought home his date, he jiggles the door handle, which makes Monica and Chandler run off into Chandler's room. Chandler kicks a circle cushion off the mat, but when the camera cuts back, it's on the mat again, and then off the mat again when Rachel walks in. A red cushion also moves.
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: In Central Perk, when Phoebe writes a note for Ross to put on his sandwich so no one eats it, she mentions that she stabbed a cop, and has a green mug in her hand. It cuts to Monica then back to Phoebe and the mug is out of her hand and she's yelling, "He stabbed me first." When it cuts back to the wide shot the mug is back in Phoebe's hand.
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: In the scene where they take Chandler to the hospital, Monica hands the bag of ice containing Chandler's toe to the doctor. The doctor is wearing gloves in one shot then no gloves in the next. This goes back and forth three or four times.
Revealing mistake: When Joey has the turkey on his head, and Chandler is by the door, look at the wall to Chandler's right. It's a "wild wall" - i.e., a movable wall put there to extend the set outward an additional few feet, since the camera normally doesn't see that far forward. The wall angles oddly away from the room, and you can see how the line and pattern of the hallway floor continues into the room.
Continuity mistake: In the opening scene where Rachel is checking the mail and is walking over to Monica's room, she has a pair of pumps on, then once she has walked in on Monica and Monica runs after her to say she was taking a nap, Rachel is wearing a pair of backless high-heeled sandals, then when Joey and Ross come in, Rachel is wearing the pumps again (we see this when Ross says "I had to talk loud because the movie was loud").
Revealing mistake: When Chandler and Monica (and later Joey) are talking on the plane on the way back from London, the little display screen (showing a map of the Atlantic and coastlines) in front of Monica's seat shows the little grey plane indicating their position and direction. The plane is facing east, as though they are travelling towards London, but they are supposed to be flying back to New York.
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).