Some years ago, in a lot of "free to play" games there was a payment method called "Paymo" that you could use to buy things from the cash shops (it was used in League of Legends, Battlefield Heroes, a LOT of MMORPG, and many more games). You had to send a SMS and it would give you a code, as simple as that.

But someday, me and a couple of friends discovered something. Thoses codes were not generated when you send a message to the service : they were all pre-generated. And because it was only using numbers and were composed of 5 or 6 characters only, you could just type random strings of numbers and and eventually you would find a good one with a random value (ranging from 1.5€ to 10€ iirc). And it was really not that hard to find them, I could find something like 10 codes in 30 minutes, sometimes less, sometimes more (at some moments you could tell there was way less codes available, we assumed it was restocked maybe twice a month). The service was not stopping you from trying until a lot of tries, and it was only blocking you for 10 minutes.

But ultimately, the system changed and it was verifying that a code was sent to your phone number. Took way more time than I expected, fortunatly for me ahah.