That is what you should be doing. I guess I am so used to people running an interview like an exam when they ask these sorts of questions that I am not expecting a logical approach from anyone anymore.

Ok. Now, I understand what you are getting at. I wonder if there is a better way to ask the question though. The question doesn't ask anything about the exhaustive nature of the testing. It only asks the reason for doing it in the first place.

Although I take issue with the last question as you can see below, I do like the way you think for the other questions. You seem to be more interested in the person's reasoning ability than their extensive knowledge of library 'X' that could be looked up in any real life situation. I run into too many "ability tests" from headhunting companies that focus too much on whether or not you've used a specific facet of the language instead of whether you can use tools to solve a problem.

You're mostly playing a word game with this. How do you make sure something works? Make sure it does what it says it will do. A bug will keep the software from doing what it is said to do. I will say that you have been out of the engineering business far too long if you actually think that your 1) is possible. Bug-free code does not exist except in small examples where no user interaction is needed and only very basic logic is used. Even then, you could find a mistake or vulnerability somewhere.