You can look straight at a real barn and still not really know you're seeing one

Philosopher Alvin Goldman imagined a driver passing through a county where, unknown to him, most of the 'barns' he sees from the road are papier-mache facades built for a film set. When he happens to look at the one real barn and correctly believes 'that's a barn,' most people's intuition is that he doesn't really know it — he was simply lucky not to be looking at a fake. The case became a standard challenge to the idea that justified true belief is enough for knowledge.

— Alvin Goldman, Discrimination and Perceptual Knowledge — The Journal of Philosophy, 1976