Even better than good error ...

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.
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 Jakob Nielsen

Quotes from the same author

The usability tests we have conducted during the last year have shown an increasing reluctance among users to accept innovations in Web design. The prevailing attitude is to request designs that are similar to everything else people see on the Web.
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 Jakob Nielsen
Popularity is the product of two factors: (a) how compelling material you offer, and (b) how easy it is to access it. Host free pirated movies and users will flock to the site, even if it's difficult to use.
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 Jakob Nielsen
Good information architecture makes users less alienated and suppressed by technology. It simultaneously increases human satisfaction and your company's profits. Very few jobs allow you to do both at the same time, so enjoy.
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 Jakob Nielsen
Minimize the user's memory load by making objects, actions, and options visible. The user should not have to remember information from one part of the dialogue to another. Instructions for use of the system should be visible or easily retrievable whenever appropriate.
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 Jakob Nielsen