I'll give you one: "Do any odd perfect numbers exist?"

You can read about it here: https://en.wikipedia.org/wiki/Perfect_number#Odd_perfect_num...

You can watch a short documentary about it here: https://www.youtube.com/watch?v=Zrv1EDIqHkY

Observability that can produce causal explanations rather than just timelines. We have great tooling for logs/metrics/traces, but very little that helps engineers understand why a distributed system behaved the way it did. Automated causal graphs for incidents still feel like an open problem.

Why should I tell you? What's in it for me?

Anti-cheat systems in multiplayer video games. It seems like every multiplayer game out there eventually gets overrun with cheaters and that cheat developers win every time.