I have this code in the end of link button click:

```
Response.ContentType = "application/zip";
Response.AppendHeader("content-disposition", "attachment; filename=download.zip");
Response.TransmitFile(Server.MapPath("download.zip"));
Response.End();
Response.Flush();

```

to download a zip file from an aspx page.  

In the previous page i set a session variable, after going to this download page and download the file, then press back i find the session=null "this happen after downloading more than 1 time", and the application\_end in global.ascx called. 

Do you know why this may happen??

Note: this is happening on all browsers, on IIS7 both local and on my server.