一个按钮该干的事被另一个按钮影响了

两个按钮button1, button2, 和一个PlaceHolder placeHolder1 都在一个UpdatePanel中。button1的click事件中有一句话 placeHolder1.Controls.Add(new LateralControl("TTTTTTTT<br/>")); button2 的 PostBackUrl设成 http://www.163.com. 该页面的form的target="_blank". 打开页面后，先点几次 button1，看到它加了TTTTTTTT在placeHolder1中。再点一下button2,发现163的页面被打开了。这时再点button1，发现它也把163的页面打开了，而且左下角也出现了JavaScript的错误。 怎么才能在点完button2后让button1还执行它原来的代码做它原来的事情。源代码：
aspx:<%@ Page Language="C#" AutoEventWireup="true" CodeFile="Default5.aspx.cs" Inherits="Default5" %><!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>Untitled Page</title>
</head>
<body>
    <form id="form1" runat="server" target="_blank">
    <div>
    
        <asp:ScriptManager ID="ScriptManager1" runat="server">
        </asp:ScriptManager>
        <asp:UpdatePanel ID="UpdatePanel1" runat="server">
            <ContentTemplate>
                <asp:Button ID="Button1" runat="server" onclick="Button1_Click" 
            Text="Button1" />
                <asp:Button ID="Button2" runat="server" PostBackUrl="http://www.163.com" 
                    Text="Button2" />
                <br />
                <asp:PlaceHolder ID="PlaceHolder1" runat="server"></asp:PlaceHolder>
            </ContentTemplate>
        </asp:UpdatePanel>
        <br />    
    </div>
    </form>
</body>
</html>CS:using System;
using System.Collections;
using System.Configuration;
using System.Data;
using System.Linq;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.HtmlControls;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Xml.Linq;public partial class Default5 : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {    }
    protected void Button1_Click(object sender, EventArgs e)
    {
        PlaceHolder1.Controls.Add(new LiteralControl("TTTTTTTTTTTTTT<br/>"));
    }
}