You shouldn't park illegally.
Sally needs to run into her office to grab her laptop, but the parking space in front of the building is metered.
Sally wants to avoid wasting time finding a free parking space.
Sally parks in the metered spot and pays to avoid the risk of getting a ticket.
Sally parks in the metered spot but doesn't pay, assuming she won't be there long enough to get caught.
A passing parking officer sees that Sally parked legally and continues on.
A parking officer happens to drive by and writes Sally a ticket.