<p>I'm trying to put in an exception in my web.config so that one page does not require authentication.  However, it still redirects to the login page.</p>

<p><strong>The question isn't how to setup the web.config</strong>.  Why?  Our system (for better or worse) has a bunch of instrumentation besides the web.config.  We have global.asax and custom HttpHandlers.  The code base isn't huge, but there's a lot of potential causes for the redirect.</p>

<p>What I do want to know is <strong>how to best determine the cause of the redirect</strong>.  Is there some way to find out what code triggered the redirect?</p>

Have you tried turning on Tracing?  That may help.
How are you specifying the page doesn't require authentication, like:
```
```
<system.web>
...
</system.web>

<location path="NoAuthNeeded.aspx">
    <system.web>
        <authorization>
        <allow roles="*" />
        <allow roles="?" />
        </authorization>
    </system.web>
</location>
```
```