I think that what they're looking for is not so much the answer you give to the question (i.e. some pre-thought and rehearsed answer) but HOW you answer the question. Does that make sense?
So if they ask "What are you best at?" you can talk about your interests and how good they make you feel, and if they're looking for a job-related answer (is this retail you're interviewing for?) then you can talk about an aspect of that, like "I find it really rewarding being able to problem-solve for customers, and that's something that I work hard at", etc.
I think it's more about them getting a feel for you as a person, rather than you giving the "correct" answers.
Also, remember that reasons for not hiring you can include things like your personality (this is not saying your personality is crap or anything, because we all know you're lovely

) may not fit in with the other people on the team, or something to that effect - like the company may not be the best fit for what you have to offer. This isn't a negative (apart from the fact that they didn't hire you lol!).
Also, if you do some research into the company then you can include that into your answers, and what's always great about doing the research is that, at the end of the interview they might ask you if YOU want to ask THEM any questions, and if you have done your research and are able to ask intelligent questions about the company, then they'll see that you are able to think in a particular way, and might think "this is the type of person we want on our team".
About the 'not making a customer happy' question, just say that there are times when a customer will be unhappy regardless of what you do, and so all you can do is give the best possible service you can, but you're only human and there are going to be times when a customer is dissatisfied no matter what you do. Show them that you're human! I doubt they want someone who's all "I am the best at everything, go me", instead of intelligent answers.
I should add the disclaimer that I've never had a job and so therefore have never been in a job interview! BUT I was talking to my dad the other night about job interviews (because I graduate soon and will have to find my first job eek!) and because he's the bomb (lol) he gave me some awesome advice, some of which I thought might help you
Also, you're still studying at uni, right? Bring that into your answers - they will see a whole human being rather than a 2-dimensional job applicant (remember how many people they probably have to interview, just be your wonderful charming self and they'll remember you for sure

).
Oh one other tip, my dad said that even mentioning something like how you enjoy for a bit of fun buying and selling stuff on Ebay shows them that you can actually organise yourself and be responsible, and can think in that way etc.
I hope some of this helped! Let us know how you get on k

Most people looking for jobs have to go to heaps of interviews before they are hired.