For a few days last summer I drove a consistent 17mph over the speed limit on the highways of San Antonio.
You see, I have a 1970 Buick that has a little knob in the center of the speedometer. The device originally could be turned to set off an alarm if you passed a certain speed, but it has long since been out of operation. However, if you don't keep the knob all the way left, it readjusts the speedometer to read, at 65mph, 17mph slower than you're really going. I don't know why.
My car had been a the mechanics for some work, and the mechanic thought he could play a joke on me by turning that knob all the way to the right. I only found out when I commented to my dad that all the cars in San Antonio seemed to be going slower lately....he then showed me the trick.
I didn't see a cop the whole time.