At the end of the Piers Anthony's "Shade of the Tree", we learn about the entity that caused the visions. We learn that the visions are based on memories of things that it had witnessed.
But the characters have one vision that doesn't seem to fit. There's a patch of toadstools that reminds the kids of little houses. And as they watch, the toadstools become little houses, and a tiny elf walks out of one. He picks up a twig, returns to his house, and lights it in his fireplace.
The book rules out the existence of such mythical creatures, so the entity had never witnessed that! Where did that vision come from?