Hello, Jay.
We have tested the scenario and had no issue with it. We've created a ASP.NET 2.0 application and ran it in both IIS and UltiDev Cassini, and had the consistent result in both: redirection worked. We have created a Defaul.aspx (and .cs) page and Error.htm. Here's what's in them:
Web.config:
Code:<?xml version="1.0"?>
<configuration>
    <appSettings />
    <connectionStrings />
    <system.web>
        <compilation debug="true">
        </compilation>
        <authentication mode="Windows" />
      <customErrors defaultRedirect="Error.htm" mode="On" />
    </system.web>
</configuration>
 Error.htm:
Code:<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
    <title>Oops!</title>
</head>
<body>
    <h1>Error</h1>
</body>
</html>
 Default.aspx:
Code:<%@ Page Language="C#" AutoEventWireup="true" CodeBehind="Default.aspx.cs" Inherits="AspNetErrorRdirectTest._Default" %>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
    <title></title>
</head>
<body>
    <form id="form1" runat="server">
    <div>
    Hello!
    </div>
    </form>
</body>
</html>
 Default.aspx.cs:
Code:using System;
using System.Collections.Generic;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
namespace AspNetErrorRdirectTest
{
    public partial class _Default : System.Web.UI.Page
    {
        protected void Page_Load(object sender, EventArgs e)
        {
            throw new Exception();
        }
    }
}
 Please let us know a little bit more about the scenario you are troubleshooting, especially what kind of error does not get redirected.
Best regards,
UltiDev Team.
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