Where to start. There's so many. I think the statue of limitations has passed already.

One of my favorite ones, I think I was about 9-10, lasted for quite some time too. There's this shopping mall in Helsinki, used to be the biggest one in any of the Nordic countries. I think it still is. It used to have dozens of phones on a wall that worked with coins as this was the early to mid 90's - cell phones only became really common around '97 or so.

Anyway, I had a routine. I used to go to McDonalds, grab some paper towels and a few straws. I'd go around the mall and cycle through the payphone stations, they were all in pairs. I would then tear small pieces of the paper and put them in my mouth to get it slightly wet. I'd then insert the end of the straw into the payphone's coin slot and blow in the small piece of paper to get it lodged in there. The optimal size was pretty much that of a pea.

What happened was that anyone who put in a coin thought the phone had malfunctioned and simply "ate" their coin. They'd then use the other phone in the station, and as I'd never plug in both of the pairs at once, they would think little of it. It wasn't as much planning on my part as it was just not wanting people to miss their calls if they really needed to make one.

Anyway, an hour or two later I'd go in and punch the phone slots They were heavy duty sturdy metal boxes. As the pieces of paper dried up it only needed a small amount of force to dislodge and for the coins to collapse through the mecanism. You can only feed the coins one at a time or they'll just come out of the "return change" slot. I'd then play at the local arcade or the poker machines with my ill-gained money, go eat at some fast food joint and so on. It wasn't just small change now in hindsight, but oh well.

Anyway, dumb as I was, I showed this trick to a few friends of mine. They then did the same. They probably showed it to a few others and so on. Few weeks later all of this income dried up and every phone was plugged, and the regular people shopping at the mall were probably complaining how none of the payphones work. I'm not sure how they discovered exactly how the trick worked, maybe someone was caught, but the mall guards afterwards were watching the payphones like hawks. As soon as they'd notice a kid stepping in to the phones they'd ask things like "Who are you calling?" and make sure they were going to actually use the phone.

From that point on there was a rule for the guards to not let kids use any of the payphones without supervision. Thus my scheme that had lasted for well over half a year came to an end.

I was really annoyed by the few friends that spread it around, but it's a small consolation that cell phones exploded in popularity shortly afterwards and the payphones died out anyway.