Character mistake: When Mike and Monica are playing ping pong, Mike's service was a fault as he released and hit the ball over the top of the table rather than behind. He then (on the same point) claims to win the point because Monica placed her hand on the table. (00:33:00)
Visible crew/equipment: In the scene where Chandler pretends that he just got home from Tulsa and meets Joey who is waiting in the hallway outside the apartment, the camera cuts between Chandler and Joey. Every time the camera shows Chandler, there is a wooden wall at the left edge of the screen with an orange tape. This is evidently part of the set, because when the camera cuts to Joey, the wooden wall with the orange tape is nowhere to be seen. (00:16:45)
Visible crew/equipment: When Chandler runs into the apartment to say Underdog has gotten away, a crew member can be seen reflected in the sun shaped mirror behind him walking through the back of the shot. It can't have been Rachel, Monica, Phoebe or Joey because Joey is sitting down and Rachel, Monica and Phoebe are nowhere near the mirror. (00:14:05)
Continuity mistake: When Rachel and Monica are discussing whether or not Rachel should meet Ross at the airport she's taking a bottle of red wine out of the refrigerator, but when she places it on the table in the next camera shot the bottle of wine is suddenly a bottle of white wine. (00:16:25)
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.
Other mistake: Mid episode when Monica turns off the cordless phone after being told by Wendy that she can not make it, the phone makes the "out of range" audio tone instead of the "power off" audio tone. Occurs directly as Monica says, "Wendy, we had a deal, yeah, you promised. Wendy, Wendy, Wendy.".