Blooper:
On the 'What if?' episode (part 1) where Rachel asks Ross when the last time they saw each other was, he says the day after x-mas 1987, but in TOW all the thanksgivings, the last time they saw each other was 1989. I guess the writers didnt think about that!

Blooper:
Chalk lines are growing! If you watch TOW the Race Car Bed, Joey writes his name on the board, and puts a line through it by mistake. The next time you see the board, the chalk line is at the end of the board, and the writing is much less messy.

Blooper:
In TOW the List, Monica states that she is allergic to cat hair. But, in TOW the Cat, she has no problem being near a cat with long fluffy hair. And in TOW the Ball, she said that the cat was making her eyes water, but it was a hairless cat.
Explanation:
I think Monica was lying about her cat hair allergy.

Blooper:
In TOW all the Thankgivings, Rachel meets Chandler twice, and in TOW the Flashback, she meets him again. When Rachel runs into Central Perk, she is introduced to him like she doesn't know him.
Explanation:
The script writers figured that Rachel never really met Chandler the first three times. The first two times (at Ross and Monica's house) she barely said anything to him, and the third time he was just a random guy that she had a fantasy about.

Blooper:
In TOW Nana Dies Twice, Joey said that he never thought Chandler was gay. But in TOW The Flashback, he clearly did by saying he was totally OK with the gay thing.
Explanation:
I think Joey was just trying to cover his mistake. Perfectly natural thing to do. :)

Blooper:
In TOW Nana Dies twice, Chandler asks if the girls thought he was gay. Rachel said she did, until Phoebe's birthday party when he spent all day looking at her breasts and she thought maybe not. Well think about this. This was the 8th episode of the 1st season. And Rachel just moved in with Monica and met Phoebe in the 1st episode in the autumn. Now, In TOW 2 Parts, Part 2, Phoebe has another birthday party! How can Phoebe have 2 birthday parties in a year?

Blooper:
In TOW All The Rugby Ross told Joey that he screamed at people, and became 'Red Ross' when they cut in front of them in the line for Dances with Wolves. Ross met Joey in 1993 (TOW The Flashback). Dances with Wolves came out in 1990, so this is a mistake, since Ross and Joey didn't know each other then and the movie has not been released since.

Blooper:
Ross also has multiple birthdays. Last count was three. In TOW George Stephonopolous, he says his birthday was seven months ago. This episode took place on October 20th. That would mean his birthday's sometime in March. Well in TOW 5 Steaks and an Eggplant, the gang celebrates his birthday in OCTOBER. In TOW the Dirty Girl (or somewhere around then) Gunther says he is collecting birthdays, Ross says, "Oh mine is December..." and Gunther cuts him off. So Ross has a birthday in March, October, and December!

Blooper:
Phoebe said that she went through a phase in high school where she thought she was a witch. But in TOW Ross' Sandwich, she said that she'd never been to high school.
Explanation:
She actually said junior high school. (And just mumbled a bit)

Blooper:
In TOW all the Resolutions, Phoebe doesn't know the names of the chords. In TOW the Baby on the Bus, however, she questions Stephanie whether she knows A minor and D.

Blooper:
In TOW the flashback, Joey was just moving in. This was set in 1993, three years earlier from 1996 which was when the episode took place. In the Thanksgiving flashbacks, the date was 1992 and Joey was there! (With a turkey on his head!)

Blooper:
In TOW All The Kips, when Chandler goes into the hall with Monica he has nothing in his hands. A shot later, an apple appears out of nowhere and lands in Chandler's hands.
Explanation:
Chandler is a paranormal being. Or maybe the cutting people should tighten up their act.

Blooper:
The porcelain dog grows! When Joey first gets it, it is up to his waist. Years later, it is almost as tall as Joey!
Explanation:
It was put on a little rolling platform so it could move more easily. In TOW Eddie Won't Go, Joey and Chandler have great difficulty in moving the dog. But look at TO On The Last Night or TOW The Embryos, and you'll see the tall dog is easily moved around. Hold on, am I actually disputing the size of a porcelain dog on a TV show? :)