One of my favorites involves an automotive paint store I worked at one summer in the 90's. They had a delivery driver, lets call him “Bud” because that was his name. 

Bud was an older guy semi-retired and very proud of the new Chevy S-10 he had purchased. Bud liked to embellish his stories and you couldn’t go 5 minutes without hearing how amazing this truck was. In particular, Bud would often brag and brag about the fuel economy of this miracle S-10.

What Bud didn’t know was that every day while he was out doing deliveries the guy sin the shop were pouring a gallon of gas into his tank. That was enough to offset his commute and keep his tank full. That went on for about a month, until it didn’t and then he spent many trips and lots of time trying to explain to the service writers at the Chevy dealer how his mpg suddenly tanked, and they explained that there was nothing wrong with his truck.

I don’t know if he ever figured out they were adding gas, but it was sure worth a good laugh to hear him bragging so hard about that amazing S-10 that uses no gas.