The Product Usability Web log has a good article talking about the gap between what the designer knows and the user is capable of in the design of applications.
The designer-user gap
This is what Jakob Nielsen (or his ghostwriter) in his Alertbox column is calls the designer-user gap. Nielsen identifies three levels of designer-user gaps:
The reality is that we are almost never the target audience, so when we say that to some executive that wants it red or blue, the truth is that they may be closer to the target audience than we are.