8. Bruce Campbell Is Secretly Mysterio - Sam Raimi's Spider-Man Trilogy
The Theory: Bruce Campbell's three characters in Sam Raimi's Spider-Man trilogy - a wrestling ring announcer, a snooty theatre usher and a restaurant waiter - are all the same person, and more to the point, they're all Quentin Beck aka the iconic supervillain Mysterio.
The theory has been circulated online since the mid-2000s, centred on the theatrical nature of two of Campbell's roles (the flamboyant ring announcer and literally being a theatre usher) as reflective of Beck's own penchant for theatrics (as a special effects whizz).
As for his taking a job at a fancy restaurant? The prevailing theory suggests this was an attempt by Beck to get closer to Peter Parker and effectively stalk him, possibly due to Peter's failure to prevent the wrestling show robbery, which in turn perhaps caused Beck to lose his own job.
The Confirmation: In 2016, concept artist Jeffrey Henderson revealed storyboards he was commissioned to create for the planned Spider-Man 4, which depicted Bruce Campbell as Mysterio, complete with his iconic bubble-helmet.
Though the ins-and-outs of the role haven't and probably won't ever be elaborated on, it's nevertheless brilliant justification for a fan theory that was undeniably pretty out-there.