Every so often you hear a story about some unwitting student solving a problem that they thought was part of their homework assignment, but was ACTUALLY an impossible-to-solve problem put on the whiteboard by their professor as an example.
Usually, the student is running late and wasn’t paying attention—or some such similar background detail that makes the story seem realistic. And the story is usually told about some famous mathematician--Newton, Einstein and Ramanujan are often mentioned.
But is this story (or some version of it) really true?
Question: Has some student accidentally solved an impossible-problem by not knowing it was impossible?
If so, who, where, when, how... and what was the problem anyway?
The perfect answer won’t just repeat another apocryphal story, but will give a credible reference or two. As Carl Sagan famously said, “extraordinary claims require extraordinary evidence.” 
Also, be sure to tell us how long it took you to find the answer AND the queries you used to drill your way towards the solution.
 Carl Sagan "Encyclopaedia Galactica,” episode 12 of “Cosmos”, original broadcast date, December 14, 1980; 01:24 minutes in. PBS.