Seeing as how fast-food systems are set up, it’s not uncommon for the cooks to get things mixed up from time to time. You might’ve received the wrong meal, the wrong portion, the wrong whatever, and the cashier or manager will have to deal with the problem as quickly as possible to maintain the flow.
However, the stresses of working in fast-food restaurants can make increase tensions in the kitchen. Worst-case scenario, your minor complaint can cost somebody their job. So, if you receive the wrong item from a fast-food restaurant, unless it goes against your diet or religion, you should probably just smile and wave.