I used to think that, if in the future (back then of course), that if we had (which of course we do now) motion-controlled games, that it would be cool.
BOY WAS I WRONG.
And now for an actual one:
I used to think that there were cheat codes in real life, but no one knew about them yet.
I've just noticed that all of mine so far are game related ones. Weird.
Well now for one that's non-game related:
I once thought that dreams were real, but just in a different universe.