"A site template has been applied to this site"

This is a very frequent error that users get especially when restoring backed up sites.  Here is the exact error message:

Server error: A site template has already been applied to this site. Once a template has been applied, the site must be deleted and recreated in order to apply a different template.

I just found a link on Microsoft site that confirms that this is a problem in the Microsoft products. Here is the link:

http://support.microsoft.com/?scid=kb;en-us;830978&spid=2514&sid=36

I would suggest users to use the second technique, that is, saving your site using "Save as Template" option. Note that you only get this error when you are trying to migrate your site from one location to another, especially if that *another* location is SPS 2003. If you are restoring the site to it's original location, then you don't get this error.

Here is a work around for this problem:

Create a new site on the target location (where you want to restore the site) and when "Select Template" page loads, do not apply a template and leave the page open. Now return to your back up site and restore it. You won't get an error this way because no template was applied to the site. 

-SSA