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).
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.
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.
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.
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.
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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.
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.
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.