Walk up to your victims car and let air out of one of their tires. Not all the air, just enough so it gets spongy.
Every time you walk by the car, let a little more air out of the tire. The idea is to make it seem like the victim has a slow leak in that tire.
Eventually the victim will take the car in and get the tire checked. Naturally, the tire will be fine.
Continue until you get tired of the gag.