Making sense of Sharepoint Errors

Below two pages describe how to enable the required keys in web.config to be able to see the actual error message in Sharepoint solutions.

Enabling the SharePoint Safe Mode Call Stack, Disabling Custom Errors and Enabiling Compilation Debugging

http://alonsorobles.com/2008/06/09/enabling-the-sharepoint-safe-mode-call-stack-disabling-custom-errors-and-enabling-compilation-debugging/

Making sense of SharePoint errors

http://www.andrewconnell.com/blog/archive/2007/02/01/5935.aspx