Unsolvable problem solved as homework.

Again not quite a prank, but nearly so.  George Dantzig (1939, not to be confused with Glenn Danzig, late 80s) was a graduate student at UC Berkeley.  Late to class, he did not realize the two problems on the board were famously unsolved statistics problems and not homework.  Stating they "seemed to be a little harder than usual", he apologized for being a few days overdue when he handed them in, completed.  And yes, this true story ended up being the basis for the movie "Good Will Hunting".