Back in the 70s when the gas crisis hit and people started keeping track of their mileage, a friend of my dad messed with his neighbor.
The neighbor was extremely anal about checking his mileage, so my dad's buddy started sneaking over every night and siphoning off a little gas. Of course, by the end of the tank, his mileage had gone to crap. After the guy got all worried, he changed his nightly sneaking by going over and putting the gas that he'd siphoned out back in so the guy's mileage started going through the roof. The guy was completely baffled.
