http://www.straightdope.com/classics/a3_199.html
Altamont Pass in the San Francisco Bay Area is a hilly region covered in windmills. Some of the windmills have three large blades that rotate on the vertical plane, and others are shaped like eggbeaters and rotate around a central vertical axis. You can see pictures at Windmills of the Altamont Pass: Photos: Highlight Thumbnails .
I (and many of my friends and family) have seen reverse rotation while driving along the highway past these windmills. It takes a little bit of concentration to “make the windmills go backward” as my family used to say when I was a kid. It’s a bit like the concentration needed to “see” the hidden “magic eye” pictures that were popular a while back.
Some of my friends have never been able to see the windmills go backward, so there seems to be some sort of knack of focusing that allows the trick of the eye to occur.
I’ve never tried to see this when standing still, only from a car. Perhaps the movement of the car along the highway is providing just enough shift in position to create the same sort of stop-action effect that was described in the article?