I do that on purpose. I wait for people to walk in front of the car, then hit the horn. You have to be careful though. Sometimes they get upset when you chuckle, so try to keep the laughs quiet.
Later on, I search BITOG to see if anyone is complaining about it.
I also make sure the car is set so the reverse lights come on when I unlock, then anyone nearby stops because they think I'm backing out. Oh, and if someone is parking lot stalking me for my spot, I often randomly walk around the parking lot to see how far they'll follow me.