Read this if you write user-facing code

http://www.joelonsoftware.com/printerFriendly/uibook/fog0000000249.html

Yes, I know you read it before - read it again, it pays to be reminded of these things every once in a while.

Oh, what's that? You don't think you have any user-facing code because you write "frameworks"? Guess what? Your users are other programmers. You still need to be mindful of these concepts.