Their mistake not mine, but a while back in my local supermarket, 6-packs of Duvel were being mistakenly scanned in at the till as single bottles, but they hadn't noticed. I noticed. €1 plus 10 cents bottle deposit, then 60 cents back from the supermarket for taking 6 bottles back. Net result...
Back in the early '80s days of classic pub video games - Space Invaders, Asteroids and that lot - once you got the knack you could usually flip a 2p piece up the coin return slot and get 3 games for 2p instead of 50p. Saved a fortune in my mis-spent student years.