Something to think about when designing a site or application is how you write your error messages. This grokdotcom article (which I can't find a byline for) encourages you to write messages that don't chastise the user for not understanding how your app works. The one comment I would make on this article is that any application developer (web or otherwise) who needs to think of the best way to phrase an error like "Phone number cannot contain dashes" should be shot.

I don't understand developers who take the time to check that a phone number has dashes (or credit card number has spaces) and chastise the user for it, rather than just stripping them out and getting on with it.