What's the meanest prank you ever did to someone's car?

Question

Bonus points if you did it without getting into their car or them even knowing it was a prank!

Answer 1

put a dead fish under the seat

Answer 2

It was actually more funny than mean!  When my younger brother bought his first car he bought a second hand VW Beetle.  This was back about 1969.  He was extremely dollar conscious and spent months searching for the best deal he could get.  Every night after he went to bed, we would top up his gas tank with fuel.  After 3 weeks with his new car, the gas needle hadn't moved and he was ecstatic at the mileage he was getting!  For the next couple of weeks, every night he went to bed we would siphon out some gas so his mileage dropped something fierce.  He nearly went out of his mind trying to figure out what was wrong with his car!  It was years later before he knew what we had done.

Answer 3

I wasn't the person who did this, but I know a guy who fed ex-lax to an english bulldog and locked the dog in an enemy's car over night.

The next morning all the widows were heavily steamed up and I guess it's obvious what happened.

Answer 4

pee on it