It happens to any customer who orders fast food from a drive-through or even a person at the checkout. You explicitly state your order — sometimes just a standard, average, unmodified order — and what you get isn’t even close.
Maybe you wanted a Big Mac combo and somehow ended up with Chicken McNuggets or maybe it’s something even harder to believe like you ordered everything the standard way and something (or maybe several things) from your order is deducted.
This can be somewhat frustrating if you discover it before you leave the store or the parking lot. Going in (or back in) to have your order corrected is a hassle and beats the fast part of ‘fast food’ but it’s much better than coming home and realizing something has been left out, replaced with something else, or just wrong.