I just saw this recently on reddit so I'll bring it here. Dr Doofenshimirtz. He doesn't "solve it" but he definitely accidentally stops the murders as part of a different plotline.
For example he creates the no-more-ink-inator that makes it so nobody can write in ink ever again because he could only write in mud when he was a child. And that inator only hits the death note before getting destroyed, making it so nobody can write in it ever again. He has no idea who Kira is and what a death note but it's stopped.