Ha, I was in Italy last year and literally tried to get a parking ticket from one of those machines for 10 minutes, without success. The only instructions were some words in Italian, which I vaguely translated as "enter the number". The number of the parking space? Nope. Maybe the number of hours? Nope. Maybe the time until I want the space? Nope. Maybe the amount of cash it should take from my credit card? Nope. Maybe it was stuck in a previous transaction and the "number" was the PIN? Maybe the machine was in some kind of admin maintenance mode and was expecting a password? Nope, the next machine asked for the same "number". Maybe all the machines were broken? Nope, there were cars with a ticket behind the front window. I talked to another group of tourists with exactly the same problem, but 7 people were not enough brainpower to figure the machine out.

While I finally walked back to car to tell the family in shame that I was, indeed, not able to use a ticket machine successfully, it suddenly hit me: the machine expects the license plate number!