Heisenberg's uncertainty principle
Max Born saw that when the matrices that represent pq and qp were calculated they would not be equal. Heisenberg had already seen the same thing in terms of his original way of writing things out, and Heisenberg may have guessed what was almost immediately obvious to Born—that the difference between the answer matrices for pq and for qp would always involve two factors that came out of Heisenberg's original math: Planck's constant h and i, which is the square root of negative one.  So the very idea of what Heisenberg preferred to call the "indeterminacy principle" (usually known as the uncertainty principle) was hiding in Heisenberg's original equations.