Spontaneous human combustion
SHC sounds like bigfoot, or just cases where someone fell asleep with a cigarette in their hand. But there are legitimate cases where someone wasn't smoking, we don't know how ignition started, or it happened fast and spontaneously (hence the name). Humans do contain sparks, fuel and oxygen, just not in mixtures that should go, "foomp!" Yet there's too many cases where it seems to have happened to ignore completely. There are hypothesis for many causes, but it's still something that appears to have happened, and we just can't adequately explain.