Probably not what you were thinking of, but the counterexamples to Euler's sum of powers conjecture springs to mind. Despite being around for centuries, it took

Probably not what you were thinking of, but the counterexamples to Euler's sum of powers conjecture springs to mind. Despite being around for centuries, it took the use of a computer to find a counterexample. https://en.wikipedia.org/wiki/Euler%27s_sum_of_powers_conjecture https://en.wikipedia.org/wiki/Euler%27s_sum_of_powers_conjec...