Mild, compared with some of the antics above, but one of our favourite tricks concerned boarding the bus. The conductor would stand on the platform at the entrance to the stairwell, in order to keep out of the way of people going upstairs or into the saloon, with ultimate ticket machine hanging down from his/her neck.
As we passed by, we would push a random lever down. hence issuing a ticket, but not tear it off. He/she usually stood there, looking absolutely bored, so had no idea who'd been fiddling with the machine, and upon giving the two bells to start, found he/she'd inadvertently issued several tickets, which, unless they wanted to pay for at the end of the day, they had to retrieve by opening the back of their machine and pulling them back through, and presumably making a note of on their waybill. All good fun.