A small, primitive village has yet to discover fire, which is a good thing because there are highly flammable (but not otherwise dangerous) gasses in the air. The heroes, blissful in their ignorance, don't know that and think it a wonderful idea to show the locals open flame.
Unbeknownest to the character, he is in the royal bllodline. Suddenly the majority of royals fall ill and die, and the hero is left with the crown, but no experience or idea of what to do.
A Deity of some sort has unleashed a plague of bookworms or something like that in the area. Almost all spellbooks are destroyed, along with bardic and historical books.
An example of a mythological worldview misinterpreting scientific practices occurred in Africa, where an aid organization, focusing on slowing and stabilizing population growth, distributed abacuses with red and white beads corresponding to a woman's menstrual cycle. Women were instructed to move one bead a day, only having intercourse on days represented by a white bead. However, the experiment failed, and the population grew in the households using the abacus. The women believed the abaci were magical, and that they would be protected from pregnancy by moving a white bead into the place of the red bead before intercourse.