Typically, the custom error page is a page of the same application. When an error occurs on one page, it sends you to this error page. The page requests are separate so the error on the bad page *shouldn't* affect the run ability of the custom error page. If there IS an error on the error page, fortunately MS thought that thru and you don't end up in an error page loop. It just does what it can. Usually the error page will be a very simple page so it is less prone to errors.
Peter
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Work smarter, not harder