C# - How To Handle PrincipalPermission Security Exceptions

I have a simple method that is secured

[PrincipalPermission(SecurityAction.Demand, Role = "Administrator")]
protected void lnkClearCache_Click(object sender, EventArgs e)
{
}

If this is clicked without the role, it generates a System.Security.SecurityException: Request for principal permission failed. as expected.

I use ELMAH to handle logging for my errors, and I have a custom ELMAH event in my global.asax to transfer to the error pages in ways that preserve status codes which works correctly.

[Code]....

This works all well and fine and redirects to my error page which works properly, however instead of displaying the content as expected. I immediately get a second request for the error page but this time using the value of customErrorsSection.DefaultRedirect that does not come from my code in any way that I can see.

As far as I can tell it's almost as if when .NET raises an exception for PrincipalPermission and then lets the entire request complete, then after the request is complete it throws away the application response and instead responds with the default custom error.

When I'm debugging I do break on 2 separate exceptions for PrincipalPermission, whether this is a just a rethrow by .NET I'm not sure but my .NET code never sees the 2nd throw, nor does ELMAH. I always end up with a single response, single error logged, but that the url that finally renders to the browser is the default url and not 403 url that I specifically server.transferred to. If I browse to a /location that is secure I properly get the 403 error page.