This is a simple digital photograph of two windmills. But when the photographer looked at the resulting image, it became a mystery, because the propeller on the one mill is not bent like that at all.
Another image was taken:
...and now not only is the propeller bent, but there's a ghostly floating propeller as well! WTF? Are our digital cameras documenting a new kind of aura??
Nope. It turns out that this can happen with cheap digital cams. Some of them rather slowly "read" the image on their sensor, line by line. If the subject moves at the right speed, especially if the speed is close to the speed of the sensor "reading", the image captured can contain these kinds of artifacts. And so the result is not what was expected.
To show the same principle, a guy turned his flatbed scanner into a camera. It slowly "read" the image it was looking at, line by line. He took a picture of his garage door opening and closing while the image was being "taken", and here's what he got:
