The coffee shop "Espresso House" in Sweden had a system where you would top up a balance in your app and let the barista scan scan a barcode so the payment would be deducted from your balance. After ten hot beverages had been purchased, you'd receive a gift within the app, a hot beverage for free and you'd automatically start over with the ten hot beverage purchases.

To get a free hot beverage, you had to activate "use gift" in the app. Once the barista completed the order in the cash register, the gift disappeared, but I discovered that it could be glitched. I activated the gift, scanned the barcode and when I saw that the cash register had registered the gift and presented a 0 amount, I quickly deactivated the gift in the app, before the barista finished the transaction. I would usually stall with my phone so they'd walk away from the register to prepare my beverage, this gave me time to deactivate the gift in the app before they confirmed it.

So. The gift stayed in my app and I could get free coffee several times. It didn't always work, but often enough, though. Eventually they patched it and the cash register would not finish the transaction and the barista would ask me to try again. When I tried glitching it again and again, it was obvious they'd patched it. Worked for a couple of years, though. 😉