Bob suffers Easy Amnesia while wearing an unusual costume or outfit. When he wakes up and doesn't know who he is, he assumes the costume is what he normally wears and tries to fill in the blanks starting from there.
Subtrope of Identity Amnesia.
- One Batman comic starts with Batman waking up, only remembering who he is on seeing his outfit. He travels through Gotham, and manages to intimidate at least one crook into abandoning his initial plans, but when he tries to stop a larger crime he's quickly beaten down. Then the real Batman shows up, explaining that the man we've been following since the beginning is an ordinary citizen (who likes to get kinky in the bedroom). He and his wife were roleplaying a Rescue Romance scenario when the man hit his head, resulting in the comic's events. Batman leaves, but asks that they stick to Superman outfits from then on.
- In an issue of Jonah Hex, Jonah borrows a clergyman's clothes to spring an ambush on some outlaws. A rockslide kills the outlaws and the preacher and leaves Jonah with amnesia. Seeing his clerical garb, and finding a letter in his pocket appointing the preacher chaplain of a nearby army fort, Jonah assumed he must be the chaplain.
- Inverted in the story "Danger From The Stars!". Superman tries to stop a meteor with some Kryptonite in it, he gets hit on the head, and falls down to a costume ball. He figures he was just attending the party, finds his civilian clothes in a pocket in his cape, and assumes his glasses are just to prevent eye fatigue. He goes to the Daily Planet, figuring a newspaper would cover an event like the ball, where he learns about his Clark Kent identity and his job, and how Superman was his "friend." He later discovers that he has Superman's powers and, instead of putting two and two together, comes to the conclusion that Superman's powers came from his suit and that Superman gave it to him to finish up the meteor job.
- One comic had Lois Lane suffer a blow to the head while dressed as a cowgirl at a dude ranch, and believed she was Annie Oakley thanks to resembling her while in costume.
- In one Superman's Pal, Jimmy Olsen comic, Jimmy is in disguise while on the job and hits his head. "Luckily", he happens to be carrying false documents that inform him that he is elevator boy Dick Hunter. When a group of crooks catch him eavesdropping and wash the dye out of his hair to confirm his identity, he's shocked to find out the truth.
- An issue of The Simpsons sees Homer suffer amnesia while at a costume party and conclude that he is Radioactive Man based on the costume he was wearing at the time.
- In one Scrooge McDuck story, Magica DeSpell is pretending to be a new secretary for Scrooge. However on her way to him, she gets hit by Rockerduck's car and loses her memory. To avoid a lawsuit, Rockerduck acts nice towards her and hires her as his new secretary. Magica takes over the role and unwillingly uses magic to help out Rockerduck's company, which starts making more money than McDucks Industries, to Scrooge's terror.
- In Santa with Muscles, Hulk Hogan pretends he's Santa Claus and gets amnesia while in the suit.
- Downplayed in Memento: Leonard is wearing a formal suit for the majority of the film, which is revealed at the end to be someone else's, which Leonard forgot soon after he donned it due to his anterograde amnesia. When Teddy asks him where he got the suit, Leonard assumes he bought it with insurance money from his wife's death.
- Buffy the Vampire Slayer: In Tabula Rasa, Spike wakes up with amnesia in a stolen suit coat with a note addressed to "Randy" in the pocket. Being Spike (and therefore British), he's furious that Giles, whom he assumes to be his father, would name him thatnote .
- Somewhat related: Anya is wearing her engagement ring from Xander, but assumes that she and Giles are together, since they happened to wake up close together and also find paperwork showing that they co-own the magic shop.
- Star Trek: The Next Generation episode "Conundrum". After an alien ship scans the Enterprise, all of the crew members develop amnesia. Worf assumes that he's the captain because he's wearing his decorated Klingon sash.
- On The Mist, an amnesiac man awakens in the woods in military fatigues, and a wallet with ID for a "Bryan Hunt". He doesn't catch on that this isn't his name until he makes it to the hospital and encounters the real Bryan Hunt, who's being treated for injuries inflicted by someone who beat him up and stole his clothes.
- A variant in Ghost Trick. The protagonist, a recently dead ghost, looks like a blond-haired man in a red suit because that's the first corpse he saw and he assumed it was his. It's not, that body in fact belongs to the Big Bad, and we don't find out until the end that the protagonist was originally a cat. The unfamiliarity with human concepts he chalked up to Ghost Amnesia was really because he was never human to begin with.
- In How to Conquer the World the unnamed protagonist hits his head at a costume party and is taken home. When he wakes up with amnesia (and a headache) the next morning, he takes one look in the mirror at his costume and the nametag which says "Doctor Evil", decides he must be a Mad Scientist and sets off to conquer the world. The fact that his few coherent memories are of a drunken rant about world domination he was making to impress a stacked blonde doesn't exactly help.
- In The Strangerhood, Sam (who, like the rest of the cast, has no idea who he is or why they're suddenly living on the eponymous street) is able to find out his name because it was written on his underwear. In the finale, it turns out that this isn't his real name (and those weren't his pants)!
- In the NMS story flashback arc in Sam & Fuzzy, Fuzzy wakes up with Laser-Guided Amnesia and wearing nothing but a bowtie. He assumes, based on his teddy-bear-like appearance, that this is his default state of clothing and keeps on wearing nothing but the bowtie for much of the comic. It's only after discovering an old jacket and talking to Hazel that Fuzzy learns that Eric used to walk around fully clothed. He's quite shocked by the discovery.
- Inverted in the Miraculous Ladybug episode "Oblivio". When Ladybug and Cat Noir find themselves trapped in an elevator with no memory of who they are, and with both their TransformationTrinkets beeping (to warn users they're about to deactivate), they decide they must have been attending the same costume party.
- In the Batman: The Brave and the Bold episode "The Mask of Matches Malone!" has Batman going undercover as a gang leader persona named Matches Malone, but after getting amnesia he truly believes himself to be a gangster.
- In Lilo & Stitch: The Series, one of the experiments afflicts the cast with Laser-Guided Amnesia. Since Gantu has a weapon and his old Galactic Armada uniform, he believes himself to be some kind of police officer instead of the show's resident Anti-Villain.
- In an episode of A Pup Named Scooby-Doo, Shaggy gets amnesia, and since he was wearing a Commander Cool costume at the time, he assumes that he actually is Commander Cool, and that Scooby is his sidekick, Mellow Mutt.