I've been involved in numerous projects where the customer didn't know what they really wanted. They had an idea that they needed something and it was left to us to define it.
Of course we knew what was required, but once we had defined it, it was too expensive. So it was chopped and changed and later changed again when the customer thought about it.
So in the end they got what they didn't really want, for more than they wanted to spend and of course late.
I'm sure this'll sound familiar to quite a few...