26. Zero Balance
My university used to have a debit card system for vending machines on campus. Your Uni ID would act as your debit card, and you could swipe them at vending machines to buy snacks.
My friends and I found out that the system could be gamed. If you only had $1.50 in your account (also the price of a 20 oz soda), you could swipe your ID at multiple vending machines that were next to each other, and then select multiple drinks/food from the machines, and only get charged $1.50.
The glitch was that account balance could never be a negative number, so if you got charged $1.50 five times at the same time, and you only had $1.50 in the account, your account would just end up with a $0.00 balance.