True story: Famous American mathematical scientist, George Bernard Dantzig recounted his story in a 1986 interview for the College Mathematics Journal:
“It happened because during my first year at Berkeley I arrived late (overslept) one day at one of Prof. Neyman’s classes. On the blackboard there were two problems that I assumed had been assigned for homework. I copied them down.
A few days later I apologized to Neyman for taking so long to do the homework — the problems seemed to be a little harder than usual. I asked him if he still wanted it. He told me to throw it on his desk. I did so reluctantly because his desk was covered with such a heap of papers that I feared my homework would be lost there forever.
About six weeks later, one Sunday morning about eight o’clock, [my wife] Anne and I were awakened by someone banging on our front door. It was Prof. Neyman. He rushed in with papers in hand, all excited: “I’ve just written an introduction to one of your papers. Read it so I can send it out right away for publication.” For a minute I had no idea what he was talking about. To make a long story short, the problems on the blackboard that I had solved thinking they were homework were in fact two famous unsolved problems in statistics.”
The professor showed them to the class as an example of unsolved problems. So, Dantzig solved them!”