Continuity mistake: In the scene where Phoebe tells the gang about her "Relaxitaxi" idea the position of Chandler's left arm changes between shots. In the frontal shots it's lying the back of the chair and in the shots from behind it's wrapped around Monica's shoulder.
Other mistake: When Ross is at Monica's party getting Mr. Green's glasses and cigarettes he bumps into Mrs. Green and ends up having to come up with an alibi for the glasses and cigarettes. He then puts the glasses on, puts a cigarette in his mouth and says 'In fact I'm gonna go out into the hallway and fire up this bad boy'. At this point he walks past Rachel but he stops still as if frozen mid-step just before the shot cuts to him walking out into the hallway.
Factual error: Joey and Chandler are supposedly crossing the George Washington Bridge but the exterior background viewed from inside the car is very obviously a repeat reel of another bridge, and their conversation goes on for far too long for the actual crossing. And the cutaway shot after Chandler is kicked out of the car shows an aerial view of the Brooklyn Bridge.
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 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.
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.
Revealing mistake: When Monica leaves the apartment and Chandler stays listening to the answering machine, check the wall beside the door. It changes the angle, as if somebody moved it (probably to hide the hall).
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: In the opening scene Phoebe and Chandler discuss how Phoebe knows Chandler is going to propose: by walking in on him whilst he was looking at a ring catalogue on the toilet. However, in the previous episode Chandler leaves the apartment right after he found out about Monica booking the wedding slot (on the answerphone) and doesn't come back until he 'freaks out' at Monica and then it is revealed that it was just put on and Phoebe is in on the plan. It doesn't add up as Chandler would have had to have been looking at the catalogue between hearing the message and deciding to propose, in the same afternoon when he wasn't even in the apartment.
Continuity mistake: The beginning to series 7 takes place right after series 6 ended, basically Chandler has only just proposed to Monica. Yet Monica's hair is visibly different; at the end of series 6 she had no fringe and a center parting, now she has a fringe. Also Chandler, is visibly thinner, with a gaunt face due to losing 20 lbs in between the two series.