Never happened to me. But I know plenty of friends who's cars have been damaged.
But as Lynne says, am I tempting fate by saying that!
I think it happens a lot when vehicles are left in foregin places, or in busy areas such as town centres because usually 'Ned' and friends are wandering and nothing better to do.
Unless of course you've upset someone. The amount of taxis I have seen recently with acid marks over the car is on the up.