I was just idly watching a few minutes of a movie called Dark Relic, which features a band of heroic crusaders returning home with a relic that turns out to be cursed. They are attacked by wolves, locusts, storms, and whatever else the devil can throw at them. At one point on their grim road dead crows start to fall from the sky, lading like big hail around and on them.
Thinking this over, it is rather creepy, and it is hard to think of anything that would have more freaked out an actual medieval person. But watching it I nearly burst out laughing, because it seemed so slapstick and ridiculous. The sound of the crows thudding against the knights' shields was particularly silly. Thomas and I started telling jokes, like, "Well it could be worse. It could be dead wolves falling from the sky." "Or dead rhinos."
Successful fantasy depends on creating the right mood, and if the necessary suspension of disbelief is lost even for a moment, disaster ensues.