<customErrors mode="RemoteOnly" defaultRedirect="~/Pages/Common/DefaultRedirectErrorPage.aspx">

void Application_Error(object sender, EventArgs e)
{
}

<customErrors mode="On"  defaultRedirect="~/error.aspx"  >
  <error statusCode="404" redirect="~/lost.aspx"  />
  <error statusCode="500" redirect="~/error.aspx"  />
</customErrors>

protected void Application_Error(object sender, EventArgs e)
{
    var context = Context;


    var error = context.Server.GetLastError() as HttpException;
    var statusCode = error.GetHttpCode().ToString();

    // we can still use the web.config custom errors information to
    // decide whether to redirect
    var config = (CustomErrorsSection)WebConfigurationManager.GetSection("system.web/customErrors");
    if (config.Mode == CustomErrorsMode.On ||
        (config.Mode == CustomErrorsMode.RemoteOnly && context.Request.Url.Host != "localhost"))
    {
        // set the response status code
        context.Response.StatusCode = error.GetHttpCode();

        // Server.Transfer to correct ASPX file for error
        if (config.Errors[statusCode] != null)
        {

            HttpContext.Current.Server.Transfer(config.Errors[statusCode].Redirect);
        }
        else
            HttpContext.Current.Server.Transfer(config.DefaultRedirect);
    }
}

protected void Page_PreRender(object sender, EventArgs e)
{
    Response.Status = "404 Lost";
    Response.StatusCode = 404;

}