Visible crew/equipment: After Marcel has humped the chair, when it cuts to Joey walking in saying that there already is a Joseph Stalin, we can see the reflection of a crewmember (presumably female) in the small mirror on the wall who's wearing a black top - and it's not Rachel, the angle is off.
Visible crew/equipment: While they're all looking at Monica's credit card statements, when Ross tells Marcel to stop humping the lamp and also when Marcel runs off to Rachel's bedroom, the reflection of the ceiling's studio lighting rigs can be seen on the TV screen.
Visible crew/equipment: While Chandler's sitting at the table waiting for the phone to ring, Joey rushes into the apartment because of telescope guy, and when Rachel walks out of her bedroom the reflection of studio lighting rigs can be seen at the top of Monica's TV screen.
Continuity mistake: When Rachel walks into Central Perk she puts her black handbag down behind the couch and proceeds to talk about Barry, but after Ross smacks Chandler's arm to prompt him to speak up, the handbag's strap appears on Rachel's wrist.
Continuity mistake: When Rachel and Ross are in the apartment with Marcel, who's still wearing the pink tutu, the bottle of wine can be seen on the side table beside the teapot, but when Ross remembers the wine it's now between the teapot and the couch.
Visible crew/equipment: When everyone walks in while Ross and Rachel are talking, just as Joey says that he doesn't need violence to enjoy a movie, in the small mirror behind Joey we can see the reflection of equipment moving, presumably a reflector screen.
Continuity mistake: When Monica takes the newsletter from Rachel it's open to the pages with engagement news, and we can see the left page has pictures as Monica reads Mindy and Barry's notice on the right page, but when Monica wiggles out of her comment about Mindy being pretty that left page is now only words at the bottom, even though she's still open to the same page with Mindy and Barry's notice.
Factual error: Phoebe holds up a bag of quarters that is almost the size of her head. This would be physically impossible for anyone except perhaps the world's strongest weight-lifters. Quarters are exceptionally heavy for their size. Two rolls weigh exactly a pound. It would take at least 300-400 rolls worth of quarters to fill up the bag Phoebe is holding, which means she is lifting 150-200+ pounds, primarily with one arm.
Visible crew/equipment: At the end, while Rachel's on the phone talking about the Visa card, in the shots facing Monica, Phoebe, Joel, and Chandler when they're playing Twister, the end of the set wall can be seen at the right side of the screen.
Visible crew/equipment: While Rachel's looking for the ring she says, "Now I'm gonna have to return the ring, without the ring, which makes it so much harder," and at the start of the next shot the reflection of a screen can be seen moving on the window behind Monica.
Visible crew/equipment: When Ross announces that Carol's pregnant, after Chandler says, "Kind of puts that whole pillow thing in perspective..." in the next shot of Ross and Monica we can see a crew member's sleeve at the left side of the screen.
Continuity mistake: In the scene in the coffee shop where Joey tells his friends about him closing down the coffee shop when he went to the audition, as Rachel says "He left work in the middle of the day to do a personal errand?" two men walk past behind the group. As she continues talking just a few moments later, the same exact men walk past again.
Visible crew/equipment: Right after the "I'll Be There For You" intro, the camera shows the view from outside the "Central Perk" café. As it does so, a couple of stage lights are visible through the upper part of the window, obviously part of the set on the inside of the building.