Public Transportation
In Sydney, Australia, we have the Opal card system for public transport. You load up the card with money, and tap it when you get on and off public transport. Now these days, per week you get a discount on all your trips after your 8th. Well. That was brought in after people exploited the previous rule.
After 8 trips, all trips were free. So people would ride the buses to and from stops, tap tap tapping away, leaving enough time between each tap for it to register as an individual trip, and after a few hours of venturing the city and tapping, they then had the rest of their weekly travel FOR FREE. The news did a piece of everyone doing it, and soon after the new rule came in.
Story credit: Reddit / wolverine-claws