More or less because you used javascript to submit the form and now that data is not available when you press the back button.
This would happen if you had no JavaScript in the page... happens to me all the time when I press back!
By pressing back, you are asking the POST request from your form to be re-submitted to the server. As the original message says, IE warns against this for POST methods only (since POST is non-idempotent) to prevent accidental submission of the data twice. If you really want to submit the data again, you can refresh while on the warning page.
Several
patterns have been developed to ensure you don't see this, the most common being the Post/Redirect/Get... in essence it uses a GET redirect so the POST request is 'lost' from the browser's history, thus preventing re-submission of the form. For example, see:
http://www.theserverside.com/patterns/thread.tss?thread_id=20936 http://www.theserverside.com/news/thread.tss?thread_id=29758 I think Bear also wrote an article on this pattern a while ago as well, but I can't find a link to it right now...
Charles Lyons (SCJP 1.4, April 2003; SCJP 5, Dec 2006; SCWCD 1.4b, April 2004)
Author of OCEJWCD Study Companion for Oracle Exam 1Z0-899 (ISBN 0955160340 / Amazon Amazon UK )