Thinking about the sneaky jokes reminded me of one my dad pulled. He had a buddy at work who got a new car and was bragging to all the guys at lunch each day about how good his gas mileage was.
I guess they all got sick of hearing it.
So my dad started adding gas to the guy's tank each day, and they would ask him how many mpg he was getting. Dad increased the amount of gas added each day until the mpg was up to 50mpg or something, this was back in the 60's before hybrids etc. (and gas cost 30 cents a gallon!!)
Anyway, after the guy had bragged on it for a while, the guys switched to siphoning out a little gas each day. After that the owner of the car would just change the subject when they asked about his gas mileage, didn't want to discuss it.
I don't think they ever did tell him what was going on, but would often ask through the years how his gas mileage was...