Maybe it was cold
It wasn't magic, that's for sure, but Jennifer Aniston's coat just was buttoned in between the takes and that could be a proof of the low temperature from the set.
What happened to the wall?
The set walls were short a lot of times and with the magic of the camera that could be solved easily. However, a few times the production's final cut wasn't too accurate and things like this could happen.
The mug changed
This one could be explained if the takes were made in different day sessions. If not how could the production change a single mug? Why did they do that for?
It took hours to make the right take
So the production only bought one candle and the used it during the whole filming of that particular scene. Joey's candle suddenly shortens from one take to the next one.
Don’t see that!
The chapter when Chandler and Joey stopped Monica from seeing the tape by throwing her on the floor, the cameras were wide enough to show one of the kneepads Courtney Cox was using.
A different color
From one take to the next one, Matt Le Blanc changed the shirt and no one in the production noticed that fact. Finally, it made the final cut and passed as a mistake.
But the door was open!
This one's from the first season of the show so maybe that's the probable explanation for this silly mistake. Why is the door in one scene open, but in the next closed?
He switched hands
What happened with the hands of this guy? Did no one notice that he changed the puppets from one hand to the other? One of the funniest takes of the lasts seasons.
Is that supposed to be Rachel?
When the production did that take again, maybe Jennifer Anniston wasn't around and they needed an extra to act the part from Rachel. Finally, fans discovered that mistake.
A mixed table
From one moment to the other, as if it was an act of magic, the breakfast table changed the distribution of the mugs on it and apparently, no one noticed that.
Rachel is desperate and crying while talking with Phoebe. When they first made that take, the paper from the kitchen wasn't a part of the scene. Until Jennifer grabbed a piece and used it.
Was that Monica?
So once again, the stunt double made it through the final cut without anyone from the production noticing it. Maybe it was a quick scene but the internet screwed them.
Once again, the closed door problem
While Monica is wiping the shirt of Richard, the door is open, but suddenly, with no explanation made, it closed. If I have to say, I would vote for a close door, because it looks better.
The dog trainer spotted
If you pay attention closely when Phoebe and Joey are scared by the dog from Frank Jr. you can actually see the head of the dog's trainer. It even moves in the same scene.
This one was actually explained
The real fans from the show know that the reason why production changed the number of Monica's apartment from 5 to 20 had to do with it being on a higher level. It had to make sense.
The moving food
During one of the dinners with the hated Paolo, that Italian guy from the building, the food on the plate moved from one take to the next one. It is a tiny movement so I'm not sure if this qualifies as a mistake.
How would you explain this?
It is impossible that a candle could cast a shadow that big on the wall. This is one of the issues when filming a scene in the dark: how to hide the lights from the set.
A cleaner take
From one take to the other, Ross' pair of pants cleans itself from the mud from the grave in which he falls while walking on the cemetery. Be careful dude!
Hide the lights!
The lighting of a set is something that producers don't like to mess with. However, do they try hard enough to hide them from the cameras? What happened with this scene?
The self-cleaning stain
That scene when they opened the door and Matt Leblanc jumped into the table with the food is memorable. But why is he sweater cleaner from one scene to the next one?
Cans aren’t the same as bottles
Joey is grabbing lots of drinks in this scene but why do they change in between takes? They aren't even the same drinks in both takes so what happened here?
Wait didn’t he hate them?
One of the things that Ross Geller claimed to hate during the first seasons of the show was that he hated ice cream because it was too cold. However here he is eating ice cream with Elizabeth.
So when is it?
So in order to be clear, when is the real birthday of Ross Geller, October 18th or in December? It's a huge mistake being one of the main characters and fans would always notice it.
Weren’t they fans of Die Hard?
If Ross, Chandler, and Joey were such big fans of the Die Hard franchise, how could they didn't notice that Elizabeth father was actually Bruce Willis? Who can explain this properly?
Where’s the ring?
Why is Phoebe eating lasagna with Monica in order to help her find the ring if she actually claimed several times that she was a vegetarian? Once again, a huge mistake!
Didn’t the know each other?
Several flashbacks proved that Chandler, Monica, Ross, and Rachel knew each other. So why does Monica introduce Rachel and Chandler on the pilot of the show? Do they have a bad memory?
The key shouldn’t work
In the last season, Rachel has a key from Chandler and Monica's apartment. However, how could this be possible if Treeger changed the locks in a prior episode? In fact, Rachel claimed that she opened the door with an old key.
How could she be surprised?
During this episode Phoebe claims that Rachel couldn't have kissed her friend Melisa, she doesn't believe she was cut out for that. However, when Rachel and Monica recovered the apartment they had to kiss in front of the guys and Phoebe was there!
It wasn’t such a magical marker
When they slept in the room and ended up marrying together the main reason was that stupid magic marker. However, the morning after the doodles actually erased and left no mark on their faces!
But Joey doesn’t know what part of the body that is
One of the first anecdotes involves Joey telling when she kissed a woman with Adam's apple. But later in the show, he doesn't know what that part of the body that is. In fact, he calls it "The Joey's apple".
This t-shirt is too big for Rachel
How Ross could be wearing Rachel's t-shirt when in another episode he wears her Frankie says relax t-shirt and it clearly doesn't fit him. Does Rachel use baggy clothes for one season?
A magic fridge door
How could Joey's fridge have a door that opens in both directions? If you remember there's a whole episode when Joey claimed he never changed the fridge and it's almost as old as he is.
Such a bad teacher
How could Phoebe teach Joey to speak French properly in that hilarious episode when she actually doesn't know a word of that language? Do you remember when she learned what "sous" mean?
Another guest star plot hole
The same thing that happened with Bruce Willis happened with Susan Sarandon and with Winona Ryder. Both actresses are in Ross' laminated list but no one recognizes them when they appear as guest stars.
We learned that Rachel's birthday is on May 5th, she tells so to Gunther. However, there's another episode when she claims that she's an Aquarius, which goes from January to February…
Seriously Ross, don’t you age?
Ross is the same age for like three seasons. He is 29 years old from season three to five and that doesn't seem to bother anyone in the production. Why's that?
He wasn’t that tough
There's an entire episode about the difficult of Chandler to cry with movies and with life in general. However, if you pay attention Chandler did cry several times during his life!
Who was the first?
There's a conversation with Joey in which Ross explains that the only girl who he slept with was Carol. However, in this conversation, we learned that he also slept with the lady who cleaned his dorm.
The correct form for air quotes
In this episode, Joey uses correctly the air quotes while telling that Chandler and Monica are "making love". However, there's another episode when he doesn't understand how they worked.
We listen to Monica when she says at the beginning of the show that she's celebrating her first Thanksgiving. However, when they remembered their worst celebration they talked about an old one in which Joey stuck his head on a turkey.
When did he leave?
Phoebe claimed in one episode that her father left before she was born. However, we learned in this episode that he sang her some lullabies and that's how the Smelly cat melody was created.
Why do they changed their minds and not called the first child Joey as they promised? In fact, the first twin kid was called Jack, after Monica and Ross' father.
Joey does share food
Even though he screams out loud that he doesn't love sharing food in this memorable scene, we actually see him sharing his food with a few friends in many episodes.
Wasn’t he allergic?
There's an episode in which Ross got stood up on a date and he orders crab cake at the restaurant. However, we learned in prior seasons that he is allergic to shellfish!
They met thanks to saltwater taffy
Even though she tells to Ross that she never tasted saltwater taffy, this candy was the actual reason why she met her navy boyfriend Ryan. He accidentally dropped a saltwater taffy on her guitar case.
When’s Phoebe’s birthday?
Even though she claimed that her birthday is in February, as she said in Season 3, in one of the lasts seasons she is celebrating her birthday with a dinner on Halloween.
Not so good for a vegetarian
Once again, even though Phoebe is a well-known vegetarian there's an episode in which she almost was capable of killing in order to get a pair of leather pants.
What happened with Joey and Chandler's bathroom? There's an episode in which the toilet is on the left of the room, but there are many others in which they opened the bathroom door and it wasn't there!
I hardly doubt that Ross could actually misspell Rachel's last name and put Greene instead of Green. How could a crew member misspell that for such a close up take?
Wasn’t that boots lost?
How could Monica be wearing the boot on this episode from season 9 when she actually lost them the very first week she bought them the prior season? They weren't that bad at all.