So, in short: incentives can have unintented consequences, as the incentives influence whatever you want to influence with them.
There are a lot of examples of this in e.g. Dan Ariely’s book and Freakonomics.
But the best example must be the bizarre 1994 footbal (soccer) match between Barbados and Grenada. Barbados needed to win with a two goal difference.
The special incentive here was that any goal scored in the extra time would count double. Now, shortly before the end of the regular time, it was 2-1 for Barbados. Imagine what happened...
(edit: added the note about the two-goal difference, thanks Hook)
-- Road sign in Griffin, Georgia, showing that sometimes it’s good to have some distance between map and area.