Your project will only be successful if the customer wants the product.
Obviously, at the start of a project there is a customer that wants a particular product.
It is up to the Project Manager via the management team to ascertain the exact nature of the product.
That is, the user expectations.
The user expectations in the form of the product specification should be clear, unambiguous and agreed by all stakeholders.
Remember, this will include performance criteria and field testing prior to delivery.
It may also include support for the product for some months after delivery.
This is common in the construction industry with contractor payments being held back to encourage completion.
When a project ends the customer gets their product.
It may well have been produced to the exact specifications but is it what the customer was expecting.
It should be if there has been good communication with the customer during the generation of the specifications.
Such a situation must be avoided at all costs. Clarify, clarify and then clarify again.
You will need to build in control measures to make sure you are still on track and that the customer has not changed his mind.
To avoid misconstruing the customer needs review the requirements regularly.