The Mystery of the Black Death-- evidence that the Black Death wasn't spread by rats (which weren't common in northern Europe at that time) and wasn't bubonic plague. It may have been some sort of hemorraghic fever.

This is of interest to ratioinalists because it's about taking a second look at whether what everyone believes is actually plausible.

Is there a closure bias which favors just choosing a theory so as not to leave open questions?