There are also optical illusions, due to some unusual interaction of light and dark, fog and reflection, nerves and synapses... We often see and hear things that either are not there or are in reality something other than they appear: the human sensory equipment is not yet perfected to an evolutionary absolute, and our imagination has always been very good at filling in blanks in a pattern, providing a narrative for experience.
When two people see the same unlikely thing, it's more likely the physical world providing unaccustomed phenomena (like those weird cloud formations earlier in this thread) than pure imagination, but the imagination still adds its own commentary, especially if another person provides a clue.
I've seen a lot of very strange things on night roads, most of which I could not be sure I seeing. Once, in broad daylight, in countryside, I very briefly saw a monkey the size of a small child, washing something at the edge of a little pond. No way! This is cold Canada; we have raccoons and bears, not monkeys. But the next time we drove that way, I saw it again: a perfectly normal macaque, perched on top of a rock, scratching its belly, contemplating life. Sometimes the weird things we see are just weird things other people do.