My better half got a new toy recently: an AR.Drone remote controlled quadricopter. I took some video of him flying it in the park with my iPhone, and noticed something odd: in the video, the propellers appeared as slowly-rotating boomerangs (in reality, they're perfectly straight 2-blade propellers). Here's a still:
It's a freaky-cool effect, and I did some research to figure out what happened. It turns out that the CMOS chip in the iPhone scans horizontally, and relatively slowly, so that fast-moving objects can alias themselves into other parts of the picture. (This is an example of a rolling shutter effect.) Propellers appear to be common culprits, and sometime the effect can be quite dramatic:
The video below explains how it happens quite well.
Update January 2016: here's a mathematical look at the curved propeller effect.