Ducky Debugging

You don't know anything about programming, but nonetheless you and your friend Bob joined the BAPC. Bob just submitted a solution to a problem, but sadly his submission was not accepted. You will help him figure out what mistake he made. In order to do so, he will explain what every part of the program should do according to him. He does not want you to interrupt him while he is explaining, but whenever he is unsure of something, he will ask for confirmation. Bob is actually a very good coder, so whenever he asks for confirmation, he is simply right. You just need to react by nodding and saying "Quack!". As long as you don't interrupt him at the wrong time, and as long as you say "Quack!" at the right times, he will eventually find his mistake and he will shout "I quacked the code!".