One of my previous heuristics was that people make more mistakes in alternative path that are usually complicated than basic flow. Here is one more example that supports that heuristic.
Today I was shopping in my neighborhood store, and alternative business flow was triggered at the cash register. Brand that owns that store usually do not have enough change coins at the cash register. So when I gave my money, cashier realised that I have enough amount in coins that she desperately needs.
Amount was 113 kunas. I gave 100, 10 in paper and 3 kunas in coins. At that moment she realised that I have 10 kunas in coins. She put aside 100 and 10, and counted 10 in coins.
You probably guess what happened. I needed to ask her to give me back my 10 paper kunas.
People make mistakes. And that is ok. This is our nature. When you are testing always be alert at alternative flows. Because people make mistakes at alternative flow with higher probability.