The One With The Jellyfish - S4-E1
Other mistake: When Chandler and Monica are sitting at the table after playing Gin, Chandler's bottle of Yoo-hoo is on the table, but the label reads "Yooh" because the brand name has been partially covered - the blue letters "oo" are yellow to match the background.
The One With The Ballroom Dancing - S4-E4
Other mistake: In the scene where Rachel is going to take out the trash, Monica tells her the garbage room is the 3rd door on the left. However, when Rachel comes back with the pizza box and Monica leaves for the garbage room, she leaves the apartment and walks towards the right - there are no doors that way, just the stairs. (00:00:50)
The One With The Ballroom Dancing - S4-E4
Other mistake: Right at the end of the episode, Phoebe's got a job interview. We see the outside of a massage parlour, but the name on the awning has been superimposed - it moves about a bit while the shot itself is still. (00:20:45)
The One With The Dirty Girl - S4-E6
Other mistake: When Rachel tells Joey he can't get Kathy another "coupon for an hour of Joey love", you can see Chandler (Matthew Perry) mouthing the words "Joey love". (00:17:21)
The One With The Embryos - S4-E12
Other mistake: Ross makes up the questions for the quiz. There are two groups of questions, one for the girls and one for the boys. Each one has four categories; there are four questions in each category. From these 16 questions, Ross asks ten. Thus, there have to be six left after the quiz. But in one group, seven questions can be seen (there are seven cards left). (00:12:50)
The One With The Embryos - S4-E12
Other mistake: Phoebe takes a positive pregnancy test right after she gets implanted with the embryos. There's no test in the world that can tell that fast if someone is pregnant - even the best tests take at least a week after conception.
The One With Rachel's Crush - S4-E13
Other mistake: Monica supposedly single-handedly changed over the guys' apartment from carpet to polished hardwood in two days. There's no way that someone of her size could have moved all that furniture solo (Rachel didn't help since she's totally surprised), stripped off all the carpet, sanded the wood, stained the wood, let it dry, and then replaced all the furniture, in two days.
The One With All The Rugby - S4-E15
Other mistake: When Ross first goes out to play rugby and goes into the scrum, just after Emily talks to the rest of the players, he jumps into the middle of the scrum where the ball should be. Instead, in the next shot, the rugby ball is to the far left of the shot, not in the actual scrum.
The One With The Free Porn - S4-E17
Other mistake: 10:50pm boarding and departure, but it is broad daylight outside. (00:09:25)
The One With The Free Porn - S4-E17
Other mistake: The new York to London time sequence makes no sense. The friends are all wearing the same clothes throughout most of the episode. Emily flies from London back to New York and Ross flies to London, most trans Atlantic flights are overnight. It's very unrealistic that Ross got to London in the same day, and she flew back so quickly after getting back to London' even though Ross had only just said goodbye to her. The characters' clothes show this is all taking place in the same day.
The One With The Worst Best Man Ever - S4-E22
Other mistake: Given her vegetarianism, and her disdain for fur in another episode, there's no way Phoebe would have wanted a pair of leather pants.
Suggested correction: It's your opinion that Phoebe wouldn't have wanted leather pants, but that's not true of all vegetarians. Natalie Merchant was known for saying she was a vegetarian but still wore leather shoes. I was a vegetarian who would never have worn fur and still wore leather shoes.
The One With Ross's Wedding, part 1 - S4-E23
Other mistake: When Phoebe tells Rachel that she thinks she can help her get over Ross, she starts by saying that she needs some photos of Ross. Rachel immediately starts walking towards the kitchen, walking right past the red box of photos on the table beside Phoebe (which she will pick up on her way back and then hand to Phoebe), before Phoebe even mentions the chocolate (and then glass of water) she also needs. It's obvious that Rachel is headed for the kitchen before she knows she needs to go there. (00:12:00)
The One With Ross's Wedding, part 2 - S4-E24
Other mistake: As the wedding begins, the first bridesmaid, Felicity, walks down the aisle with a man. He is not part of the wedding party but is wearing the same attire as the best men. He is not seen again throughout the wedding. The only two men in the wedding party are Joey and Chandler. (00:38:20)
The One With Ross's Wedding, part 2 - S4-E24
Other mistake: In the episode with Ross's wedding to Emily, Joey is talking on a cell phone with Phoebe as he walks down the aisle. Joey holds the phone out to Ross saying 'it's Phoebe', and one can clearly see that the phone's screen is completely blank. The phone isn't even turned on. (00:39:20)
The One With Ross's Wedding, part 2 - S4-E24
Other mistake: When Emily first walks in wearing her wedding dress, there is a distinctive difference between that dress and the one Monica picked up from the store a few episodes earlier and tried on (yes, more than that she is also wearing a jacket). There is a band of material at the top of the dress when Emily is wearing it that was not there when Monica wore it.
Suggested correction: Phoebe explains before she takes the first pregnancy test that her body's "always been a little faster than Western medicine". It's a deliberate joke.
A deliberate joke it may be, but we all know Phoebe believes a lot of silly things, and this is one of them. It really didn't make sense for her to get a positive result that fast.