I have been having difficulty in determining why my Application_Error event in global.asax is seemingly fired twice.

I've created a sample web application to demonstrate what happens. There is NO other code (or pages) beyond a default.aspx, error.aspx, and the global.asax.

Default.aspx has this:
-Very simple code to do nothing more than throw an error.

    protected void Page_Load(object sender, EventArgs e)
    {
        throw (new DivideByZeroException());
    }

Error.aspx has this:
-Grab the exception out of the Session object and display a message.

    protected void Page_Load(object sender, EventArgs e)
    {
        Exception ex = (Exception)Session["Exception"];
        something.Text = ex.Message;
    }

Global.asax has this:
-Get the last error, put it in session and then redirect to error page.

    void Application_Error(object sender, EventArgs e)
    {
        Exception ex = Server.GetLastError().GetBaseException();
        HttpContext.Current.Session["Exception"] = ex;
        Server.ClearError();
        Response.Redirect("~/error.aspx", true);
    }

When debugging all goes according to plan for a little while:

1. The exception is found and tossed to Application_Error.
2. Application_Error puts the exception into the session.
3. The ClearError is called (I have read that this is necessary in order to store things in the session in Application_Error. Is this true?).
4. Response.Redirect fires... and then...
5. Application_Error is called again, but a null reference exception is thrown when putting ex into the session (presumably because there isn't an error pushed into ex -- I suppose it was cleared by ClearError).

So the questions:
1. Why the second visit to Application_Error?
2. Is Server.ClearError really necessary?

The web application has literally no other code in it other than what you see above, so there shouldn't be anything else generating an error. I'm sure nothing else is generating an error because on the 2nd visit to Application_Error the exception is null.

Any thoughts?