While I was working at a customer site, my customer wanted to change the URL of the portal. Instead of using Alternate Access Mappings, I extended the web application with a new URL and deleted the older web site.
After a while, we wanted to backup the web application, but we got "sharepoint KeyNotFoundException" exception. The info for the exception was "The given key was not present in the dictionary". I have searched a little bit and found that this error was because of my extending and deleting the old web site operation.
I deleted the web application without deleting the content database using the Central Administration web site. I created a new web application with the new URL, but used the old content database. The problem was over :P