Continuity mistake: When Monica is in Richard's apartment she gets all worked up, so Richard gives her a hug to calm her down. She goes into the hug with her arms curled up in front of her face, but when the camera angle changes, they are at Richard's back.
Continuity mistake: In the episode with Chandler in a box, when Rachel is showing Ross all the things she had kept from their relationship in a shoe box, her hair is tucked behind her ear in most shots, but when she says "I keep the gifts that matter", it's all loose, then goes back to how it was in the following shot. Also in the shot before that line she's pointing the fossil at Ross, but when it cuts to a front view she's pointing it upwards.
Continuity mistake: In the one with Rachel's babyshower, Monica needs to ring Rachel's mom to invite her. She says that she has her number in her phone book and walks over to the phone - we see her start to look at it, but when the angle changes she's got the phone and is dialling.
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: In the One in Barbados (part 2), just after Monica bashes her hand against the table, the shot of Chandler asking if she's OK shows Monica raising a finger right at the edge of the screen. We then cut to Monica, who's cradling both her hands together.
Continuity mistake: In the scene where Chandler sits on Joey in an attempt to regain possession of his seat, the shirt sleeve on his left arm becomes unrolled and slides down. When he stands up, the camera cuts to face Ross while he is giving out directions, leaving Chandler's back to the audience. Chandler's sleeve is now rolled back up. The sleeve remains rolled for the rest of the sequence.
Revealing mistake: Whenever Monica's apartment is shot from the point of view of the balcony looking in, fake walls have to be put up across the room to cover the open space (since there is no wall there usually). When the friends are looking out the window at Ugly Naked Guy and his girlfriend in this episode, you can see that the rear wall does not extend nearly far enough - there is a large black space behind Monica.
Continuity mistake: When they are talking about sperm donation, Joey is eating a cracker with jam on it. Between shots it instantly rotates in his hand. You can tell because the jam goes from being on the top to the bottom side, and because of the way he is holding it you can see there is only jam on one side.
Continuity mistake: When Chandler's boss is slapping all the guys on the butt, on the side of the table closest to the camera there is a tablet with a pen on it. When the guy comes back in to get his briefcase, the pen is no longer there.
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.
Visible crew/equipment: While Monica and Rachel are talking about Chandler's "jollies to Jaws" Rachel's wearing a colorful flowery top, and after Joey walks in just as Monica takes her seat at the table in the closeup, it's Jennifer Aniston's stand-in, wearing a blue top, who is seated beside Monica. (Viewed in widescreen).
Visible crew/equipment: After Ross goes in for his disastrous tan, when it cuts to Joey and Rachel walking into their apartment the hole (used for a camera angle) in the wall can be seen at the right side of the screen. As an aside, when Ross leaves Monica and Chandler's apartment later just as Rachel walks in, that hole is seen in the hallway and it's covered up.
Continuity mistake: It's obviously December, with the set decorations and Rachel's reference to sitting on multiple Santas' laps because she is randy from her pregnancy hormones. The characters establish that it's Rachel's fourth month of pregnancy...but she became pregnant about six weeks before Monica and Chandler's May 15 wedding. Her fourth month would have been in July, not December.
Continuity mistake: There is a scene in the guys' apartment where the phrase on the Magna Doodle keeps changing. Joey is complaining to Chandler about having to stay awake all night for a sleep study. Rachel and Ross then enter the apartment (at different times) and Ross announces he is getting married to Emily in a month. In some shots during the scene the Magna Doodle reads, "get out!" in other shots it reads, "Poop." (Widescreen).