In this electronic age when money comes down to bytes in someone’s computer, security is more important than ever. Major security holes are easily created when a mistake is made sometimes even from just one person. Often when these flaws are detected it’s already too late and nobody know how much damage that bug has caused.
For example as this article explains, ATMs have a so-called “operator mode” from where the operator can change settings such as the which bills are located in which slots. This mode is accessed when six digits are typed into the keypad. When a company that sets up ATMs forgets to change this code from the default one (123456) which is also stated in the manual – anyone can make the ATM machine think that it’s giving you $1 bills, when it’s actually giving you $20s.