Gojko Adzic – The Mythical Customer Problem:

"Customer is a role, not a person. Very often it can’t be a single person – a domain expert that should ideally be on site is seldom the executive project sponsor who has the authority to approve cost or change plans. One group of people will know how the process works at the moment but another will have a vision what the new system improves. In any non-trivial business specialists focus on particular parts of their domain, so any single person will not know all the details of all the problems."