<customErrors mode="On" defaultRedirect="~/Error.aspx"></customErrors>

<script RunAt="server">
    void Application_Error(object sender, EventArgs e)
    {
	Exception ex = Server.GetLastError();
    if (ex != null && Session != null)
    {
        ex.Data.Add("ErrorTime", DateTime.Now);
        ex.Data.Add("ErrorSession", Session.SessionID);
        HttpContext.Current.Cache["LastError"] = ex;
    }
    }

</script>

protected void Page_Load(object sender, EventArgs e)
{
    if (IsPostBack) return;

    if (HttpContext.Current.Cache["LastError"] != null)
    {
        Exception ex = (Exception)HttpContext.Current.Cache["LastError"];
        if (ex.Data["ErrorTime"] != null && ex.Data["ErrorSession"] != null)
            if ((DateTime)ex.Data["ErrorTime"] > DateTime.Now.AddSeconds(-30d) && ex.Data["ErrorSession"].ToString() == Session.SessionID)
                Label1.Text = ex.InnerException.Message;
    }
}

Server.Transfer(String.Concat("~/Error.aspx?message=", HttpUtility.UrlEncode(ex.InnerException.Message)))

void Application_Error(object sender, EventArgs e)
{
    HttpContext.Current.Cache["LastError"] = Server.GetLastError().GetBaseException();
    Server.ClearError();
}

protected void Page_Load(object sender, EventArgs e)
{
    if (IsPostBack) return;

    if (HttpContext.Current.Cache["LastError"] != null)
    {
        Exception ex = (Exception)HttpContext.Current.Cache["LastError"];
        if (ex != null)
            Label1.Text = ex.Message;
    }
}

Server.ClearError();