Other mistake: After Ross breaks the radiator in the women's apartment, it gets so hot in there for their Christmas party that all the attendees strip down, some to their underwear. However, when Rachel answers the door to let a couple guests in, her nipples are erect, which would indicate the temperature inside is cold. (00:15:16)
Other mistake: When Rachel arrives at Central Perk, Monica introduces her to everybody, including to Chandler, as if they don't know each other. In later episodes, we learn that Chandler met Monica and Rachel a couple of times when they were in high school (including when Monica cut his toe off). Chandler even kissed Rachel once, and since Chandler always says that he hasn't kissed many women, he should have perfectly remembered Rachel, and Monica should have remembered that the two of them knew each other. (00:04:40)
Other mistake: When Phoebe and Ross are at JFK, some of the people who are at that airport are the same people who are also at Newark. It's particularly funny to see the people behind Ross and Phoebe at JFK when Phoebe claims they're on their honeymoon, and when it cuts to Rachel at Newark as she's looking for her boarding pass, those same people are there too.
Other mistake: The layout of the hotel surrounding the main cast's rooms as displayed during the sequence where Monica, Chandler, and Phoebe eavesdrop on Ross, Charlie, Rachael, and Joey is physically impossible. Ross enters the room Monica, Chandler, and Phoebe are in through the same wall they are using to listen in on Charlie and him, though while the camera is in Ross and Charlie's room, no door can be seen in that wall. Furthermore, a hallway can be seen through the doorway while Ross holds the door open, implying that the two rooms do not share a wall in the first place.
Other mistake: Chandler tries to open the door that leads to his room, but Joey has sawed the door in half so only the upper half opens, and Chandler then falls over the bottom half. He has to hit it with some force to fall over it, but it doesn't budge an inch. (00:00:50)
Other mistake: When Monica realizes that the cookies were the Nestle Toll House recipe, Monica tosses the yellow bag of Nestle brand chocolate chips to Phoebe, and the brand name has been partially covered so it looks like "I Iest" and the Toll House logo on the side has also been partially covered.
Other mistake: In the opening scene, while Joey's eating the bowl of Frosted Flakes the box is on the counter, and the brand name "Kellogg's" has been painted blue with its white line extended and covered with a sticker. At the end of the episode, when Joey's eating the Cocoa Puffs the box on the counter has the brand logos for General Mills covered with stickers on the front and sides.
Other mistake: When Rachel's watching the soap opera on TV while babysitting Marcel, she and Marcel are sitting in the middle of the couch, but in the closeup of Dr. Francis we can see the reflection of the empty couch at the bottom of the TV screen.
Other mistake: This is the episode where a guy across from Monica's apartment is spying on them. Joey says he caught him spying into his and Chandler's apartment yesterday. This is impossible considering the layout of the building i.e. the fact that the only part of Joey and Chandler's apartment which faces toward the spying guy has the stairwell and hallway blocking it from view (not to mention the protruding part of Monica's apartment). There is another window in Joey's apartment but it faces completely the wrong direction. The bathroom has no windows. The angles just don't add up. (00:00:40)
Other mistake: When Rachel leave her message on Ross' answering machine she mentioned the name Michael after 17 seconds and "I'm over you" after 25 seconds. When Ross listen to the tape he asks "Who is Michael ?" after 8 seconds. Rachel flips the phone out of his hand after 18 seconds so he couldn't have heard her "I'm over you." (00:10:35 - 00:12:50)
Other mistake: When Chandler, Monica and Erika arrive back at the apartment after sightseeing, Chandler enters the apartment from the left (past where their apartment door is) rather than from the right, as if he came up the stairs. He obviously is entering from the stage and didn't take the time or cue to appear to enter from the direction of the stairs.
Other mistake: When Joey comes out the bathroom in the girls' apartment with a turkey stuck on his head, the bathroom floor has the black and white tiles. But that floor wasn't there until Joey put it there in Series 3, The One With Frank Jr. So in the flashback the bathroom floor should be white tiles. (00:04:45)
Other mistake: In this episode we seemingly understand why Ross and Monica's parents favor Ross. In one scene at Monica's apartment Ross says that it is because their parents thought Judy was barren. This would make sense except that in TOW Rachel's Book, Jack and Judy tell Monica and Chandler that they had to get married because Judy was pregnant. Judy was using a diaphragm as birth control and Jack's dog ate it. It doesn't make sense that Judy would use a diaphragm if she thought she was barren. This was not news to Monica since she said she had heard the story about the dog eating her mother's diaphragm at her 5th grade Halloween party. (00:04:15)
Other mistake: In the scene where Rachel is talking to Chandler in the hall, after she says "the 4th of July", it cuts to a shot from behind Rachel's head. If you look on the right hand side of the screen, you can see a piece of wood in the foreground. This probably came from Joey and Chandler's doorframe, through which this view would have been filmed. In addition, this door was supposed to have been closed throughout the entire duration of this scene. (00:17:55)