In Taipei (臺北) my guide book said that the bus system had this bizarre arrangement where sometimes you paid when you get on and sometimes you pay when you get off, with no method of telling when you did what.
The first time I caught a bus it was immediately obvious when you did which. Clearly the guide book writer had been poor at research. The buses have the character on a electric sign above the door. 上 (up) means you pay when you get on and 下 (down) means you pay when you get off.
My bus was 下. I travelled to the National Palace Museum (國立故宮博物院), quite a trip out of town. There was man with his son and daughter. As I walked to the front of the bus, I found that he had already paid for me. He did not want my money and hiding behind a lack of common language waved me on.
On the return trip my bus was an 上 bus. I paid when I got on the bus. It also became clear that, assuming this pattern worked across all bus lines then it was a load control system. People paid at the part of their journey away from the center of town. Hence the buses had shorter stops in the congested inner city and were quickly loading and unloading where it was needed.
NB: It seems that one of the people in the hostel followed me to the bus stop. When I got their my bus was just leaving. I started calling out “Wait!” but the futility of this, plus the fact that it was in English, caused me to stop before I got the the “t”. They said, you do not say wai to the bus. (Wai is what you say as hello on a mobile phone.)