Paul Stork from Mindsharp just posted this to the sharepoint discussions yahoo group and I thought it was worth writing down as I am sure that I will need to use this one day.
If you ever receive the classic sharepoint error:-
An error has occurred.
Please contact your system administrator if this problem persists.
and would like to have some actually useful info on what has happened try the following. (Quoting Paul directly here)
To get a bit more info make the following
changes to your Web Front end server in the Web.config file of one of
the WebApplicaitons generating the error.
Search for CallStack= and change the setting from false to true.
Search for CustomErrors= and change the setting from Off to RemoteOnly.
(This means that the CallStack will only show on a browser on the server
console. Remote users will still get the SharePoint error page.
Then do an IISreset.
Finally, go to the WFE server console and browse to one of the pages
generating the error. Instead of seeing “An error has occurred”. You
will see a full stack dump and exception statement for the error that is
occurring. That may give you enough information to find out what is
going on.
Gavin's Sharepoint Blog 