Continuity mistake: Just after the opening titles cut to Central Perk, four people can be seen having a conversation through the window. In the next shot, there is one person sitting next to the window and a waitress has appeared out of nowhere.
Continuity mistake: Chandler and Monica are having dinner with her parents. He asks Jack how he and Judy got engaged. He starts, "Well I'd gotten Judy pregnant..." In an earlier episode they talk about how Jack told Monica's friends a "scary story" about how Judy got pregnant because her method of birth control, her diaphragm, had malfunctioned (damaged by a dog). In the rest of the show, however, we are continually hearing that the firstborn, Ross, was "a medical marvel" and that they thought Judy was barren. This apparently is the main reason why they love Ross more than Monica.
Continuity mistake: When Rachel suggests to Joshua that they should get married, he says "What?", and his hands are out in front of him. The camera cuts to a wide shot and his hands are now on his thighs. Cut back to Joshua and his hands are now out in front of him with his fingers interlocked.
Continuity mistake: When Rachel and Tag are on the balcony and Tag says, "What did Joey say? I like you back?" he is standing on the balcony with one hand in his pocket and the other in front of him. As the camera angle changes, Tag is now in a different position, with both his hands behind him, leaning against the balcony before he moves forward.
Continuity mistake: When Rachel says, 'One can never be too Alan, ' we see the stick pom-poms that Ross is holding are up facing Monica, but in the next shot the pom-poms are facing the floor, but after Chandler says 'I personally could have a gallon of Alan' in Monica's closeup we see the stick pom-poms are up again facing Monica.
Continuity mistake: In the previous episode, Joey tells Chandler that the bathroom in his new apartment has mirrors on both sides of the toilet. But in this episode, when we see the toilet from the side while Joey gives the gang a tour, it is clear that the wall is devoid of mirrors.
Continuity mistake: When Chandler and Rachel figure out that the chairs were switched they both head over to Joey's apartment. When thet enter the apartment, the new black chair can be seen with tags attached to the top and left side. When the scene cuts to Joey sitting down the tags are all missing from the chair.
Continuity mistake: Apparently Ross is Monica's older brother but in "The one where they're going to party", Joey states that the three men are all the same age ("we're 29, we're not women"), and in the one with the birth Monica says she is 26 and Joey says he is 25. If Joey is younger than Monica, and Ross and Joey are the same age, then there's definitely a mistake somewhere.