Used to work at starbucks like 7 years ago and they used to print these receipts where if you filled out a survey it would give you a 6 digit code which you could then exchange for a free drink.
However when you gave in the receipt with the code we would just toss it in the garbage and then give the free drink. So over a shift I would just keep all the survey receipts when people didn't want them, write random numbers on them, then keep a wallet full of free coffee. Since I already got tons of free drinks, I would give them to homeless people and explain it was good for a free coffee. Probably gave away like 100 free drinks.