This practical joke is best done to people who are truly worried about their car,
truck, whatever. The type that parks their car 20 hectares away from the mall
to avoid it getting touched by human kind.
When your buddy leaves the car, get your friend to place a note on the
windshield that reads "Sorry, about the scratch. We will take care of any
damage." and add a fake phone number. Then as you come out with your buddy from
the mall, and they read the note, let the search for the non-existent scratch
begin.
You can help by pointing out "potential" scratches, watching him or her run over
to see, only to realize it's just dirt.
This one can done from afar so that you can have practical fun watching the
owner of the car survey for damage rather than you helping out, can be even more
hilarious.