Designers are not users.

What we learned is money doesn't grow on trees.

If your users have many questions, it's a failure of your primary site design. It becomes not so much customer support, as much as customer complaints.

...the book is a manifesto to make the Web atone for the sins of computers and regain a level of simplicity that can put humanity at peace with its tools once again.

Even better than good error messages is a careful design which prevents a problem from occurring in the first place. Either eliminate error-prone conditions or check for them and present users with a confirmation option before they commit to the action.

A general principle for all user interface design is to go through all of your design elements and remove them one at a time.

Designers are not users.