As an apprentice TV repair man I was often introduced to “Mr. Megger” the insulation tester. First time it was connected to the workshop door handle when I was late, second time it was connected to the other end of a mains lead they asked me to put a plug on. Also had match heads put in the end of the soldering iron which explode when you turn the iron on.
Best one was the Honda Acty van joke. It only had key locks on the drivers door and rear tailgate. So, if you parked it tight into the corner of the car park, exited by the passenger door locking it by pushing the button down, the only way to get in was by bouncing it out of the corner to gain access to one of the key locks!