This is the original OOPS! Bad Parker Card. It's a nice and effective way to let people know they made a mistake. It's better than writing an angry note. It actually gets people to move their car and change their behavior.
 
The text of the card reads: OOPS! You accidentally took up two spots with one car. Please be more considerate next time. After all, we are living in a society. We all share the same space.
 
$5 for 100 cards.