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A strobe lamp can be used to seemingly stop a fast moving object when calibrated.
Commonly used in quality assurance during production to inspect otherwise non-observable assembly line activity.

What causes this effect in observations?

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Well, it doesn't really stop the motion. If you time the strobes to coincide with the revolution period of a wheel, the wheel will make one exact revolution between strobes and will appear to be stationary. This is called sampling in signal processing (you might want to read about the Nyquist sampling rate).

On a different note, one has to be careful not to undersample, otherwise motion artifacts will occur. For example, the fact that stagecoach wheels turn backwards in western movies is because of the fact that the sampling rate of the movie (24fps) undersamples the motion. This effect can be very important, e.g. if you are a doctor evaluating the video of a beating heart for diagnosis, very high frame rates are used in this case.

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To make the explanation clearer, I would add that the flash has to be very brief, so the object cannot move much while illuminated. –  hdhondt Jun 19 '12 at 2:04
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