I'm going to be a bit pedantic and say fault and mistake are not equal in the message they convey. Hans says he has 'made a mistake' which is not the same as saying 'I'm at fault.' In the end it might be proven that you are at fault but until then all you know is that something went wrong, isn't working, etc.
I think that’s the kind of pedantry that is relevant in that case and in designing your product and customer support tree.
Friends of mine hearing him would say, He never says what the mistake is precisely, but there’s always the option that it was booking a flight with that airline.
