Basic Trope: A character gets mistaken for a poor impersonator of himself.
Straight: Superbob enters a "Superbob Cosplay Contest" and loses.
Exaggerated: Superbob loses the contest to someone with a cheap junk costume that look nothing like his.
Downplayed: Suberbob wins second place.
Superbob's costume is tattered and dirty from all the superhero work, while the competitors' costumes are spotless.
Superbob is a great superhero, but a shoddy tailor, and his costume is badly made. Meanwhile, his fans are better at sewing.
Superbob's normal costume has been stolen, and he's using a less flashy backup costume... meanwhile, the competition is won by the crazed fan who stole Superbob's costume to win all cosplay contests.
Superbob operates only at night, and no one ever saw him clearly; thus, people think his costume looks completely different than what it really looks like.
Superbob's costume is rarely actually seen by the judges, mainly because his exploits take him beyond the reach of cameras, like the moon, so they know what he's supposed to look like but not the exact details.
Alice the Really 700 Years Old witch keeps trendy with the times, slowly updating her "outfit" with the current style. Because people expect her to dress as she did long ago they think the actual Alice is a "modern redesign" of the witch everybody knows.
Superbob wants people "know" for sure that he's not Superbob by getting his costume and persona totally wrong.
A very well-costumed fan gets mistaken for Superbob. Hilarity Ensues if the mistake is made e.g. by a supervillain out for revenge.
Superbob gets recognized as the real deal, but he tells the others, à la Einstein, that he is not Superbob.
Superbob gets recognized halfway through the judging.
Someone in a crappy-looking Superbob costume is implied to be Superbob, but turns out to be just a fan.
...But still loses to someone, simply because their costume looks cooler.
...except the fan is really Superbob in disguise, because no one would suspect someone in such a shoddy costume of being the real Superbob.
Parodied: Bob the Giant Man-Eating Worm from Another World attacks people. They still refuse to believe that he's the actual Man-Eating Worm, and instead confuse a chubby kid in a cheap costume with the real deal because he looks more "authentic" to them!
Zig Zagged: Superbob apparently wins the contest... but then the judges reveal that it's just a consolation prize, and the actual award goes to someone else. Who then reveals himself to be the real Superbob; the one who got the consolation prize, and the one whose adventures we've been seeing from the beginning of the episode, was just a good cosplayer.
Averted: Superbob is declared winner. His win helps reinforce the masquerade that Bob is not Superbob.
Enforced: "Last rule, the best costume must be sewn properly."
Lampshaded: "Guess they are more of a Superbob than me."
Sought by a powerful enemy, Superbob hides among a group of costumed fans, knowing that the enemy will never consider him the real Superbob.
"At least they're better at sewing."
Defied: The contest rules specifically prohibit Superbob from participating.
Discussed: "It'd be funny if the actual Superbob took part in our contest, no?"
Conversed: "Heh heh. That gag where they think he's just someone in a cheap costume always cracks me up."
Deconstructed: Superbob realizes that the civilians love him not as a person, but as the image of him they crafted in their own minds - and that they only like him as long as he lives up to their image. This drives him into depression.
Reconstructed: Superbob decides that if his sole purpose is to be a symbol, he might as well be the most heroic symbol ever, and regains belief in himself.
Played For Drama: The whole situation carries a moral about how different heroes actually are from our views of them.