You get a nice error that says, “A new guard page for the stack cannot be created”, well easy fix. I could be long winded, but the issue is simply this, you caused a stack overflow.
How you say? Well, check to see if you are using a Server.Transfer that transfers you to a page that then Transfers you back and forth, or even to yourself.
Have login code that checks to see if you are logged in? If not, then it bounces you to a login page. Maybe you put it in a master page. Well, if the login page uses the master page, it will just keep transferring itself to itself and you blow the stack.
Response.Redirect will give you a different error as the browser only jumps a low number of redirects before giving up. Server.Transfer uses the internal stack, so it blows up on the server itself.