1. The service industry is difficult. You're not the only customer in the restaurant, and your order isn't the only one they have to worry about. They have to do a ton of work all at once, so don't get pissy when your order is messed up, just politely point it out and they'll fix it.
2. Most people in the fast food industry that aren't just working to save for college aren't great with multitasking. It doesn't necessarily mean that they're stupid, but they aren't brain surgeons, and if their job for the rest of their life is working at a fast food restaurant, don't expect a brain surgeon.
3. That job fucking blows. It is not fun, it is not easy, and it is not rewarding. Do you expect them to put forth 110% into every, single, solitary order that they receive? The amount of work they do and the amount to money they receive is just depressing, so don't be shocked if they're not pouring their heart into your cheese burger.
4. It's a small mistake made, and it's made over an even smaller thing. Just calm the fuck down.