Pareidolia is the word for the phenomenon of seeing unplanned, or unintended images in artwork. Typically faces, the most famous example is probably the Man in the Moon.
Leonardo da Vinci wrote this about pareidolia,
"If you look at any walls spotted with various stains or with a mixture of different kinds of stones, if you are about to invent some scene you will be able to see in it a resemblance to various different landscapes adorned with mountains, rivers, rocks, trees, plains, wide valleys, and various groups of hills. You will also be able to see divers combats and figures in quick movement, and strange expressions of faces, and outlandish costumes, and an infinite number of things which you can then reduce into separate and well conceived forms."
Indeed, uneven surfaces, textures, broken brush strokes all seem to add to the effect.
Below are a couple of examples of my paintings that I've been told have unintended images in them:
Can you spot the pareidolia? I've been told that the top image has a man's face with a beard and white hair, in the clouds, sort of like Santa Claus. In the next painting I've been told there are fighting beasts with gnashing teeth, eyes, and guns. But then, someone else saw flowers and stars.
Like Rorschach blots, when it comes to pareidolia, state of mind seems to have a lot to do with what one does or doesn't see.
For fun, here are some links to more obvious examples:
Holiday Cats! Thanksgiving Cat
15 hours ago