[AntonyGarand]

This brings back memory: This was the case for a gold-buying website for the Runescape game in the 2000s. You could edit your cookies or other front-end facing information to change the price of items in your cart, so you could buy gold or items for much cheaper than the market rate. At some point, while the vulnerability remained, they started cancelling orders abusing this and manually checking the orders.

I think you could still find some old youtube videos or threads on obscure forums with enough digging about that topic, that's how I learned of it initially.

So this was a real thing!