We had several experiences of overpaying in the bus. The last time was in Hongkong when we had no coins and I had to insert $10 in the fare bin. There is no change so you need to have the exact fare if you don’t have the Octopus card. That also happened in Singapore when we had to pay more due to lack of coins. It’s all right because those are only coins but if it happens often then it’s not all right anymore.