Red Crucifix sighting in 774 may have been supernova
Short version: a guy in California reads the recent story about a spike in carbon 14 in trees around 775AD, but no one knows of a supernova report around the time. He Googles a bit and finds a possible candidate.
It's not quite up there with my putting two and two together about 18th century resuscitation methods, but it's close. :)