you'd be suprised how much can go wrong. it doesn't matter how simple the order is. remember that these are 16 year old kids working for minimum wage. i think they could care less if you aren't happy with your food.
but since we're talking about mcdonalds here...
when i was about 10 years old, we stopped at a mcdonalds next to fort sam right off of austin highway. i got a chicken sandwich meal. when we got home, i bit into the sandwich and there was a hair. i opened the bun to make sure there wasn't anymore, and i found a wad of black greasy hair the size of my fist. there was also a foot print on the chicken patty.
of course, being 10 years old, there wasn't anything i could do. i showed the sandwich to my dad and he brought it back. he got into it with the manager there after the manager told my dad that we fucked with the sandwich so we could try and get a free meal.
then a few years ago, my brother went to a mcdonalds drive through, and the bitch that took his money was in a pissy mood and she threw my brother's money on the ground. the manager tried to start a fight with my brother after he complained about the incident.
needless to say, mcdonalds will never get a dime from me. hell, i don't even eat fast food anymore, and i'm better off for it.