Captain Awesome is vulnerable to a specific species of animal. His weakness would be laughable in areas where the animal thrives, but understandable in locations where such a species is scarce.
Justified: Captain Awesome has a severe allergy to plastic in general and plastic forks are some of the most common forms of easily acquired plastic.
Captain Awesome draws his powers from plastic forks, giving him not a Weaksauce Weakness, but a weaksauce power source.
Captain Awesome is totally immune to plastic forks (and only plastic forks) in any situation in which one could harm him, however rare such situations may be.
Captain Awesome's weakness is Francium, one of the rarest elements in the universe which generally only last for 30 minutes before decaying into other elements.
Turns out the plastic fork was laced with cyanide.
Alternatively: Captain Awesome deliberately spread rumors that he was vulnerable to plastic forks to distract his enemies.
Captain Awesome isn't really vulnerable to plastic forks.
But Captain Awesome can easily resist the cyanide, but not when he's suffering from the plastic allergy.
Alternatively: Now that Captain Awesome's false allergy has been exposed, his enemies will no longer be distracted from finding out his true weakness: Bread.
Well, at least he isn't vulnerable to plastic. Turns out he's vulnerable to the mere shape of a fork.
It wasn't plastic fork it was the corn on it.
Kryptonite Is EverywhereUp to Eleven. Captain Awesome's secret identity works at a plastic fork factory, putting him a hair's breadth from death every day. All his villains have plastic in their armor, and they pointedly observe they are trying to uphold standards by only fighting/doing crime with non-lethal... plastic weapons. Oh, and his nemesis? Plastic Neptune, who wields a giant plastic trident.
Captain Awesome is weakened by exposure to oxygen, so his amazing powers are really useless.
Captain Awesome treats his fork-problems like a traditional allergy, complete with epi-pens and inhalers.
Zig Zagged: Captain Awesome freaks out at the sight of a plastic fork but does nothing to him. Thinking he is over his plastic allergy, he picks up a second one, but it turns out this second plastic fork was not hypo-allergenic, and he promptly has a severe reaction. But turns out to be instead a reaction to Plastic Neptune's allergies-beam. But then it turns out that the plastic fork caused Captain Awesome's immune system to go crazy, making him vulnerable to Plastic Neptune's allergies-beam.
Averted: Captain Awesome is not driven off by any mundane, easily accessed substance.
"Crap, Captain Awesome is a bit too awesome. He's making all the villains look laughable. How do we bring him down to their level. *frantically searches office, reaches into fridge* A FORK!"
"Geez Captain Awesome, you can shrug off meteor impacts and eat plutonium, but a plastic fork makes you cower in fear".
"Of all the things that could weaken me, why did it have to be something as commonplace as a plastic fork?"
Invoked: From the Diary of Senior Sinister "Grr, All superheros have weaknesses, and I know I can find Captain Awesome's. I'll try everything from Aardvarks to Zygotes".
Exploited: Captain Awesome's enemy wears a suit made of nothing but plastic forks in order to defeat him.
Defied: The villain menaces Captain Awesome with a plastic fork, he cowers in fear and hears out the villain's monologue and plan. Once he knows how to stop it, he then casually slaps it out of his hand and asks if he really believed he would come unprepared. Turns out he's wearing his special hypoallergenic uniform that makes him immune to plastic forks.
Conversed: "You know, it's a wonder there are any super powered people fighting or doing crime at all. They have so many weird allergies they ought to live in a bubble".
Discussed: "No, there is no cosmic balancer or something bandying out random weaknesses to superpowered people. If you run into someone who about to toss a truck at you, do not pelt them with mistletoe, just run".
Captain Awesome only has a slight allergy to plastic. His deep rooted fear of plastic forks actually comes from before him gaining his powers, when his abusive step-father molested him with them.
Alternative: His glaring weakness causes him to become the laughingstock of the superpowered world and he is forced to retire from crime-fighting because he has such a glaring weakness.
After Senior Sinister finds out the Captain Awesome's weakness is plastic forks he tells his goons to attack Captain Awesome with plastic forks.
But Captain Awesome swore that he would stop people like his step-dad, so he made a deal with a supernatural being that allowed him great power at the cost that he be driven off by what scares him the most.
Alternative: After dropping out of supering, he realizes he can still make a difference, he just needs to account for his glaring weaknesses. So he develops a suit that protects him from the forks or helps with disaster relief.