Pouring sugar into a car's gas tank is thought to be a petty yet effective means of sabotage. As parodied on The Simpsons, it's the preferred revenge method of estranged (and deranged) spouses and partners.
The idea is that the sugar causes the car's engine to permanently seize up, resulting in a costly trip to the mechanic. The guys at Project Farm recently put this mischievous theory to the test. Here is the video.