http://www.youtube.com/watch?v=wyMjtq3IodU
A quick google search and this comes up. I think that describes sweep error the best. That car is only 10-15 feet away, getting a reading of 9 mph. If that car was closer to 200 feet away, the speed reading would be much higher.
I wonder if the training that these officers go through is consistent with the manuals for these devices. The manuals say the best way to reduce sweep error is to use a tripod. And even that doesn't get rid of error completely.

