Explaining errors is one of the basic concepts in usability for the web. It is as simple and obvious as telling the user what was wrong and how to solve that problem.
"It is important that error messages are clear…"
There are many ways of explaining errors, like going back to the web form marking the erroneous fields. It is important that error messages are clear, avoid things like "error 4055".
Digg, the social bookmarking website, displays from time to time an error message at the end of the submission procedure. I don’t really know why this happens because the error message is not clear, no solution is proposed and there is no entry about that on the FAQ section. The user experience could not be worse in this situation.