There is honestly no excuse for screwing up orders, and people should be held accountable. Because you know Taco Bell screws up your order constantly, check the bag BEFORE you leave the store. (I do this all the time in CVS even though the workers get annoyed because one time, I got the wrong medicine and another time the box was right but it was empty.) When the order is messed up, MAKE A SCENE. Nothing will get attention faster.
The Manager will come out and this allow others (while they are in the store) to know they run the risk of the same problem. You may find you get results. I know I did even though it wasn't a fast food place.
When my grandfather was alive he had an iron deficiency and once a week we had to go to this one place to get his paper work so that he could go to the hospital to get the blood transfusion. Like clockwork we would go there, get the paper work or they claim they would fax it to the hospital so we would leave. When we'd get to the hospital they would either not have the paper work or the paper work we had was wrong. I couldn't take it anymore and went back to the place and started screaming my head off in the lobby that I had never seen so many incompetence workers piled together in one place in my life and I was so sick of their idiocy. Of course, everyone in the lobby, as well as the nurses behind the desk looked at me in shock. But the following week, and every week after that until my grandfather passed, we never had another problem with them.
It's sad, but sometimes you can't play the nice guy. Sometimes only being the rude jerk gets results. You make a scene the next time you come in they will remember you. And when you get that bag, ask them with an angry tone if it is correct. They will tell you yes, but OPEN IT IN FRONT OF THEM to make sure. It's your money, you should get what you pay for.