Gag Haircut

Essentially every person has a bad hair day, it happens. Unfortunately for these characters their bad hair days just happen to be a whole new level of bad. The character will find themselves in a situation where they need a haircut...only for the haircut to go horribly wrong. But typically the bad haircut will only last one episode and it'll be back to normal by the next episode. Usually overlaps with Traumatic Haircut except while the haircut will be traumatic for the victim, it's always Played for Laughs, so it could be seen as a subversion of the Traumatic Haircut. The situation quite often involves getting gum in the hair. Variations can include bad dye jobs which may or may not result in actual hair getting cut to solve the problem.

Examples of Gag Haircut include:

Anime & Manga

Kazumi Magica plays with this trope for the better: Kazumi goes to get a haircut, but the stylist cuts off most of her hair. In a sense, this is an Establishing Character Moment since, metaphorically, cutting off hair is a sign of a new start in life. Of course, most, including her friends, don't like this new look since her long hair was shown on the cover.
A version not Played for Laughs occurs in Wandering Son. The thirteen or so year old protagonist wishes to get a pixie cut he saw on a female model, however due to the fact he's beginning puberty it turns out looking masculine instead of feminine. He eventually begins growing it out.

Film

In It Takes Two, Amanda spits gum into Clarisse's hair so the next time we see Clarisse, she has a truly awful hairdo.
The 10th Kingdom has Virgina get Rapunzel Hair from a spell and Wolf takes a magic axe to it, leaving her with a very short hairstyle. One of the rare occasions where the haircut isn't fixed with a Hair Reboot.
Mr Troop Mom one girl gets gum in her hair that must be cut out. Her hair still looks fine afterwards.
Ramona & Bezus has the former making a tiara out of bur seeds, them getting stuck to her hair and having to be cut out.
In Serial an angry hairdresser chops off Tuesday Weld's hair when she brags about her new man, who happens to be the hairdresser's boyfriend.

Literature

Anne of Green Gables was annoyed about her red hair and bought some hair dye from a peddler on the road hoping to turn it black. It ended up green and had to be cut off. The hair grew back though.

Live Action TV

The CBBC series Mike and Angelo had Katie's hair ruined after an unusually large trumpet blew her into another room so Angelo's machine tried to fix it for her. She ended up looking like a man.
Happens to Monica in the season 2 premiere of Friends. She tries to get Phoebe to give her Demi Moore's hairstyle but Pheebs mixes her up with Dudley Moore. The end of the episode implies she's going to give Julie the "Roddy McDowall cut" (instead of Andie MacDowell).
The five-minute short series Bus Life has Jess get gum in her hair and must be "operated on".
Sonia in Eastenders was unlucky enough to fall asleep while her daughter was playing hairdresser.
Seinfeld made a whole episode out of Jerry getting a bad haircut.
Marshall in How I Met Your Mother got a two-fer when Lily's cousin gave him bleached blond highlights an hour before his wedding. He then ended up shaving those off to give himself an even worse haircut.
An episode of Lizzie McGuire involved a prank war between Kate and Miranda and one of Miranda's pranks involved putting green hair dye in Kate's shampoo.
Sabrina the Teenage Witch cast a spell to make her hair fall out (hoping to get her aunts' attention) and they hired a specialist (played by Ru-Paul) to give her a purple version of Marge Simpson's hairdo. She then gave Salem the same hairdo at the end of the episode.
The 1950s B/W series I Married Joan has an episode where Joan goes to get her hair styled for a college reunion, and ends up with an awful new do called the "panda cut".
An offscreen variation happened in Charmed. Actress Rose McGowan dyed her hair red between seasons and the characters joked in the season premiere that a potion blew up on her and turned it red.
Played with in USA High. Bobby discovers a talent for hairdressing and starts doing all the girls' hair. When Lauren finally gives in and gets him to do hers, he goofs and dyes it bright green. When he tries to fix it, Lauren runs out screaming with her hair seemingly falling out. Then she pulls of the wig and reveals she'd just been playing a joke.
On The Big Bang Theory, Penny persuades Sheldon to cut his hair. All goes well up until the very end, when, while she is trimming the back of his neck, a ticklish Sheldon moves and Penny accidentally shears a bald spot off the back of the head. He doesn't notice it right away, but Penny muses that she might have to move when he does.

Newspaper Comics

An early storyline in Calvin and Hobbes has Hobbes cutting Calvin's hair. He keeps getting it uneven and trying to fix it, until Calvin is left with no hair at all. Bill Watterson has commented that one of the few times he ever laughed at his own comic was after drawing the results of Calvin's haircut.

Professional Wrestling

Both Kurt Angle and Molly Holly lost special gimmick matches, which meant they had to have their heads shaved completely bald. For weeks after the shaving, they both wore outrageous wigs in their matches and it would be a Running Gag for the wig to get ripped off during the match.
Behind the scenes, Dolph Ziggler had to cut off his blond locks because he got gum in them.

Puppet Shows

An early three-part sketch on Sesame Street had Ernie playing barber and cutting off all of Bert's hair.

Western Animation

The aptly titled Recess episode "Bad Hair Day" followed the gum plot with TJ's attempts to fix it and giving Mikey an awful haircut. They lied to say a famous band had the same cut. The fun starts when the other kids hear this and decide they want the haircut as well.
In an episode of Dexter's Laboratory, Dee Dee accidentally cuts one of her pigtails off and gets a serum from Dexter to grow them back. The hair ends up taking over the house.
In 6Teen, Caitlyn ends up with a red mullet after "cheating" on her hairdresser.
One episode of The Simpsons had Lisa getting gum in her hair and finally going to the hairdresser after trying everything to get the gum out.
Rugrats: Chaz lets Chuckie cut his hair in hope of showing him it doesn't hurt. Chuckie goes a little overboard.
Jeannie (Hanna-Barbera's version) goes to a hairdresser and tells the guy too late to not cut her ponytail. She and Babu spend the rest of the episode trying to grow the hair back. It gets so long it runs down the street by itself and is mistaken for a fuzzy red serpent.
Happens twice in Sabrina the Animated Series where Harvey's imaginary friend Mort has fun messing up Sabrina's hair. He ends up shaving it all off and growing it back five seconds later.
- Secondly, two faeries persuade Gem to cut her hair into a mohawk, saying it's the latest trend in Canada.
In the episode of The Powerpuff Girls called "The Mane Event", Bubbles and Buttercup accidentally rip a chunk out of Blossom's hair and do a scary tidy-up job. Even the Monster of the Week finds it funny.
In American Dad, Hayley dyes her hair green at a Green Party rally. Stan cuts it off while she's asleep.
Inverted in The Weekenders episode "Tish's Hair", where Tish gives herself a new hairstyle with fuzzy buns on either side of her head. Her friends hate it and think she'll become a laughing stock...but then everyone shows up to school with the same hair.
In an episode of Arthur Francine and Muffy decide to cut all their hair off during a head lice scare. They have some fun by sneaking into the boys' bathroom and pretending to be boys.
In an episode of The Cramp Twins Dorothy takes Lucian and Wayne to the barber's and becomes obsessed with cleaning it, eventually washing all his tools in a freakishly strong floor polish. He then uses the tools to cut her sons' hair and it ends up half falling out.
Happens to Kitty (and usually someone else) in almost every episode of Tuff Puppy.

Webcomics

In the comic Ozy and Millie there's a running gag about Ozy (an anthropomorphic fox) ending up losing all his hair due to diferent circumstances.
This has happened several times in Stubble Trouble, twice to Rosie when she was cooking.