I think I once posted a bonus question that had no known solution, but I added the caveat not to spend too much time on it. Don't remember what it was though and SN search is not helping me find it.

This reminds me of that story they used in Good will Hunting about George Dantzig:

In 1939, a misunderstanding brought about surprising results. Near the beginning of a class, Professor Neyman wrote two problems on the blackboard. Dantzig arrived late and assumed that they were a homework assignment. According to Dantzig, they "seemed to be a little harder than usual", but a few days later he handed in completed solutions for both problems, still believing that they were an assignment that was overdue.[4][6] Six weeks later, an excited Neyman eagerly told him that the "homework" problems he had solved were two of the most famous unsolved problems in statistics.[2][4] He had prepared one of Dantzig's solutions for publication in a mathematical journal.[7] This story began to spread and was used as a motivational lesson demonstrating the power of positive thinking. Over time, some facts were altered, but the basic story persisted in the form of an urban legend and as an introductory scene in the movie Good Will Hunting.[6]

Maybe I should.

But I'm sure people here who'd have an inkling of chance at solving one of these likely would recognize it as a Millenial problem or other unsolved problem.