A session will timeout after, by default, 20 minutes. If you aren't using sessions, then turn them off, since they consume resources on the server. If you do need sessions, and the timeout is too fast, you can modify it in the IIS Manager dialogs (Properties/Home Directory/Configuration/App Options/Session Timeout).
You can also modify it on an individual page by setting the Session.Timeout property to the number of minutes you want to timeout in.
Thanks Xoc but what you are saying is not what I mean, nothing to do with sessions. Indeed, you could have a look what I am saying with webmasterworld itself.
You have to have Windows XP to get the error:
Just go to Site Search and do a search for instance for Google, click on the second page of the Results Page (Results Pages: 1 2 3 4....) at the bottom of the page. Then press the back button of your browser. This is the error I got and only happens with windows XP.