The authors of The Pragmatic Programmer about users not knowing what they need:
Dave Thomas: Tying in to what Andy said earlier about software having a Heisenberg effect, where delivering the software changes the user’s perception of the requirements, almost by definition, your target is moving. The sheer act of delivering the first release is going to make the user realize, “Oh, that’s not quite what I wanted.”
Andy Hunt: Or even worse yet, “Oh, that’s exactly what I wanted. But now having seen that, I’ve changed my mind. I’ve learned. I’d now like to do this instead, or this in addition.” Just by introducing the software, you’ve changed the rules of the game. Now the user can see more possibilities that they weren’t aware of before. The user will say, “Oh, if you can do that. What I’d really like is if you could do this.” And there is no way to predict that up front.