When I was in secondary school, we used farecard (a stored-value magnetic card) to pay for our bus fare. The system was straightforward: board the bus at the front, insert the farecard into the device, choose the bus fare needed, and collect the bus ticket. The amount would be deducted from the farecard. When the amount in the fare card ran low, the device would reject the card and the passenger would need to pay by cash.
During that time, bus fare cheats would usually cheat by paying the lowest amount. If they were caught by the bus inspectors (those dressed in white), they would need to pay the extra amount needed. At that time, there was no penalty nor fine imposed.
There was another method that some cheats used. Usually this method worked on a crowded bus. They would purposely be the last to board the bus. As the bus was crowded, they would be standing near the door or at the doorstep of the bus, then they would insert the farecard into the device pretending to pay the bus fare but the device would reject the farecard due to insufficient amount in the farecard. The bus driver would be unable to see them because of the crowd. At the next stop, the cheater would alight.