Empty Guarantee
Back when Hollywood Video was around, they would guarantee new releases to be in stock. If they rented out all of their copies in the store then they would give you a voucher for a free rental.
I was in college at the time and would go into the store near my college campus at around 10pm on a Friday night. There was simply no way for them to have any copies left by that time, so I would collect whichever vouchers were available. Rinse, repeat. Cheaper than redbox.
Story credit: Reddit / Ragina_Falange