Character mistake: After the guys have beaten the girls in the quiz, Rachel refuses to move apartments and Monica says: "If we had lost, we would have made them get rid of the birds." But they did lose. That's why the guys have got their apartment. She should have said if we had won.
Character mistake: When Monica and Mike are playing table tennis, Monica puts her hand on the table and looses the point. Mike then does the same thing. Surely Monica, of all people would notice that as it would have given her the point. We also see her looking at Mike at the time. (00:33:40 - 00:39:25)
Character mistake: In the early years, the gang are talking about their pets dying and their parents saying they sent them to a farm and Ross thinks that actually exists. He said that they sent their dog Chee Chee there and HE could run about chasing animals. Then in TOI Massapequa, Monica is preparing her speech and she tells Chandler about Chee Chee and said SHE died when Monica was in school, and when Parker asks whose dog it is, Monica says HE's my old dog. Then during her speech, Monica says HER and Nana are gone. (00:15:30)
Character mistake: In one of the scenes where Phoebe is teaching Joey guitar, she asks him to demonstrate the "Iceberg" chord. To refresh his memory, Joey refers to the chord as "G Sharp." In reality, the only way to play a G Sharp chord is with a barre chord, and as such the fingering is not exclusive to it.
Character mistake: Both lightning rounds at the end of the quiz are significantly longer than the 30 seconds that Ross claims they are. (Even though he is supposedly using a stopwatch to time them.) The guys' lightning round is actually 43 seconds long, and the girls' lightning round is 45 seconds long. (00:14:20)
Character mistake: When they are about to start the game, Ross declares that "the first team to answer the most questions wins." This is a nonsensical statement. By that logic, the first team to answer a question correctly would be the winner. It should be either the team that has correctly answered the most questions at the end of the game wins, or the first team to correctly answer an X number of questions.
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)
Character mistake: When everyone hears that Chandler has a third nipple, even Ross is surprised. He's known Chandler for many years - and they were college roommates - so Ross would certainly know this fact.
Character mistake: Even though both Ross and Rachel have cell phones with them and use them for other calls, it never occurs to Ross early on and as he's frantically racing to the airport with Phoebe, to call and tell Rachel that he loves her or at least to say that he is on his way to tell her something important before her plane takes off for Paris.
Character mistake: Ross tells Chandler to run a "post pattern to the left." (And Chandler quickly agrees.) In football, this is a type of passing play where Ross would hold on to the ball while Chandler runs straight down the field immediately after the snap, eventually veering diagonally to the left once he's a ways down field to receive a pass from Ross. The play that they actually run is completely different. It's a running play known as a sweep. (A toss sweep to be precise.) There is no possible way that anyone either making play calls or running them would ever confuse a post vs. a sweep. (00:10:10)