the nicest, or perhaps the most intriguing, truth i've found is that the fundamental axioms of certain things we learned in university - axioms that put limits on what can be done in the real world - were all wrong. I recall one specific sentence in one of the books we used, where it said "we assume x to be true for this entire book, because it is!". This wasn't limited to our curriculum - these axioms are held to be true in the entire academic field dedicated to this subject, and i've read literally hundreds of papers "proving" it is true.
in case i sound like some lunatic, this is regarding efficient-market hypothesis. Turns out the market + the academics theorizing about it, are all human, are all fallible, prone to biases, and are dead wrong about many things. Thus for anyone with enough creativity, ingenuity and guts it's all out there for the taking.