My cousin just messaged me and asked me to make him a discount code for an online retailer.
THATS NOT HOW IT WORKS.

@Alice they responded with “Disappointed.”

What kind of coder can't make codes?

Jokes on you.
There was a local new pizza delivery place and they might have rushed a bit to go online..
Why?
Client side validation and calculations. Right click total price, inspect, change input to 0.01€ and pay with paypal. Payment went through, order was placed and delivered. Even had 0.01€ printed on the receipt.
Did it two times before they fixed it, only one dude thought that price had to be a error on their side. Yeaaaahh.. right :D

A friend of mine was looking through the source code of a webshop once and stumbled upon a list of all the discount codes, hardcoded into the JavaScript.

@Olverine I had a CD of a certain software package that had its serial numbers inside a txt file. You could write something like 11111-22222-33333-44444 inside of the file and it will work at the moment of the installation :p

@GodHatesMe NOPE