Okay, aside from the usual Santa Claus, Toothfairy, Easter Bunny type stuff. (Sorry if I ruined anything for you there 1499... :p )
What crazy stuff did you believe when you were younger that you don't now? (but let's not bring up religion stuff if you don't mind.)
For me it was all about Ninjas. I believed Sho Kosugi was the greatest Ninja ever. He had the best movies.
Now I know that he's a typical Japanese actor who is proficient in showcasing his martial arts for the camera...and became typecast as the archetypical Ninja in Hollywood due to a few early roles.
I believed you could learn to become a Ninja by purchasing ninja magazines and studying the step-by-step photos. My belief was reinforced by the introduction to one of them (Titled: 'Your Complete Guide to Ninja Training') naturally. It assured the reader that all content was reviewed by current masters of Ninjutsu for accuracy. It advised that you seek one out for Ninja training, but if you couldn't, then the magazine was your next, best option. As long as you were trying...you were walking the path of the Ninja. Being a ninja is all about your mind set and your heart, not your capabilities. (We see this same philosophy with the current crop of wanna-be real-life mandos).
I had an authentic ninja outfit; tabi boots and all. Self-made shuriken... I'll stop there before I delve into anything more questionable. Let's just say when I put it on, I don't go to cosplay at conventions.
I beleived that the ninja were the ultimate warriors of Japan...and that Samurai must have just hudled in their master's castles, praying the ninja didn't come to kill them tonight.
Now I have studied Japanese history...and I chuckle at my youthful exuberance; the reality that the ninja were nothing more than a class of soldier whose abilities and roles changed over many centuries; from barely-trained peasant suicide-soldiers, scouts and spies, to highly trained Samurai who had been given permission to abandon conventional honour in order to carry out specific acts of subterfuge.