“What’s more, the price is calculated by the client(!) by multiplying the unit price of whatever is being purchased by the quantity being ordered.”
MTG Arena’s server checked the price by repeating the client’s math, overflow and all, in a 32-bit int. Order 21,474,837 packs at 200 gems each and the total wraps around to almost nothing. A new account’s 250 free gems bought over $20 million in cards, and maxed-out packs refunded gems for the next set. Wizards of the Coast patched it after the report. The validation copied the bug it was supposed to catch.