This seems like an unlikely urban legend but it actually happened at UC Berkeley in 1939. A doctorate candidate was late for class, and found two equations written on the chalkboard. He thought they were homework, and turned the solution a few days later.
The reality is that they were a couple of math theorems for which there was no proof yet. His solutions were the first! The student was surprised a few weeks later when his professor excitedly told him he had written an introduction for his paper and was ready to publish it. The man said this was the first time he'd had an inkling that they were anything special.
This story eventually became legend, with the obvious moral that you shouldn't think of anything as impossible because it restricts what your mind can do to solve even the hardest problems. Read more about it at the source!