How to nicely inform to the user that an unknown error has happened?

There are several guidelines for error reporting, that are usually based on giving to the user useful information when he or she does something wrong, but to give this kind of information you need to be handling the error and know that it can happen. There are also tons of articles about designing 404 error pages. But, what can you do when it's a new, unhandled error provoked by a failure in the software?

Are there some guidelines about how to nicely report totally unexpected errors in a web site as an unexpected error 500?

What header message should be shown in that case? something like "Sorry, an unexpected error has ocurred" would be enough?

What information should be given?

Should it have mechanisms to help to report the failure to developers? Which ones?

Solution

First thing to keep in mind when an error happens : do not frighten or confuse your users.

Your error pages should integrate into your site in a seamless way : similar layout, same colors. It should be obvious that this page is part of your site and is a consequence of a previous action.
As Matthew Wilson and codymanix said, you should not be too technical. Error messages must be clear and intelligible. If you do not know where the error comes from, just say it.
If the error happened during a transaction of any kind, you should inform the user about the consequences. If an error interrupts an online order, you should be able to tell if the transaction has been carried out or not.

Now you have told your users something bad happened, it is time to inform the developers about it. There are plenty of solutions around, including :

Custom error log
OS-provided error log (such as Windows' Event Log)
E-mail alerts