We humans are pattern recognising creatures. Pareidolia is a psychological phenomenon where the watcher or listener perceives an image or sound to be significant.
In this video that I took while on holiday in Jamaica this year, it looks like a demon appears in the window glass. In reality it is no more than a weird looking random stain left by condensation. The 'demon' first seems to appear because the light refracting through the glass renders it invisible until I walk around to capture it at the right angle with my handy HD camera.
The Skeptick's Dictionary has more information on pareidolia
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