The One With The Chicken Pox - S2-E23
Factual error: Ryan is a LCDR submariner, who had been on a sub deployment for the last two years. but at no time does he wear his dolphins pin. A true submariner would sooner forget his skivvies than his pin.
The One With Ross's Library Book - S7-E7
Factual error: When Ross shows Chandler the book, he claims it is in "the biggest library on campus" at NYU, which would be Bobst library on Washington Square Park South. Later, in Central Perk, when Ross tells Rachel and Joey about the people "making babies" in front of his book, Joey says, "Wait a minute. Fifth floor, against that back wall?" Bobst's fifth floor is conference rooms only; there are no stacks on that floor. Joey might have been confused, but Ross should know better, being a professor and having just visited that location. (00:08:30)
The One With The Free Porn - S4-E17
Factual error: We know that Phoebe is three months pregnant, because it's the day after the party where she told Joey that she was three months along. But the ultrasound shows three little lumps that in no way resemble a three-month fetus. An actual three-month fetus is the size of a jellybean.
The One With The Ball - S5-E21
Factual error: There is no way that a cop and a masseuse could have afforded an apartment in Brooklyn Heights, even in the '90s. (Brooklyn Heights was unaffordable in the '80s.) "Rent control" doesn't explain it either, as a vacant apartment - which is what they are looking for - would be raised to "market rate" if a rent-stabilized tenant moved out. This can't be written off as a character mistake: Gary is not naive or uninformed, and his accent makes clear that he is a New Yorker.
The One With The Fake Party - S4-E16
Factual error: Phoebe says in the show that she's three months pregnant. Not only is she way too big for that, but the fetus also kicks for the first time. Women don't feel fetal movement until 16 weeks at the earliest, and for a first pregnancy it may not be until 25 weeks or so.
Suggested correction: Phoebe's size could partially be explained in that she was carrying triplets, though Lisa Kudrow was pregnant in real life, and was further along than her character. Her pregnancy had to be written into the show and the gestation timelines did not match up. Phoebe feeling "movement" could just be her imagination, as her character is supposedly somewhat psychic and often perceives things that are not necessarily discernible to others or isn't even real.
The One With The Giant Poking Device - S3-E8
Factual error: The "very long poking device" that the friends fashion out of chopsticks would never actually work. It has vulnerable points every half-foot or so (the length of the chopsticks) and after a certain point would just break or sag under its own weight at one of the vulnerable points.
The One With The Screamer - S3-E22
Factual error: Rachel's date yells at the theater patrons who are in the gang's seats; one of them has D13, and he screams "you're surrounded by even numbers, does that give you some clue?" However, the establishing shot shows they're at the Lucille Lortel Theater, which has odd-numbered seats on the left - the side they're sitting on. It is also a relatively small house and doesn't have a D13; the outermost seat in that row is D9. (00:04:50 - 00:05:30)