Flamethrowers

Flamethrowers weren’t banned in warfare because it’s bad to be let on fire. They were banned because when sprayed into enclosed spaces (bunkers, caves, and tunnel complexes) you are essentially gassing everyone inside with carbon monoxide. There were caves in the Pacific theater of World War 2 where dozens of Japanese soldiers were killed by a single flame thrower operator. Many if not most wouldn’t even have burns, some never even seeing the flames. Apparently the Americans didn’t actually learn how they'd been killing them until after the war.