back button gets Warning, page expired

When a user fills in a form and clicks the submit button and is on the new page, if they click the back button in the browser, they get a warning from Apache: "Warning: Page has Expired The page you requested was created using information you submitted in a form. This page is no longer available. As a security precaution, Internet Explorer does not automatically resubmit your information for you. To resubmit your information and view this Web page, click the Refresh button. " While the previous form will be displayed if they click Refresh, no one ever reads this. They just see the page expired and believe the info is gone. So, my questions are: 1) Why are they getting this message? When I do just a little test page with one field in the form, this doesn't happen. The back button just goes back to the form with a warning. What triggers this warning? 2) How can I do this so that the warning does not appear. So that the user just goes back to the previous form, retaining the info in the form? 3) Alternatively, does anyone know how to change the warning message in Apache? If I could display a different message, I could make it more clear to users. I know the Apache directive for changing an error message. However, the syntax requires the 3-digit error code. This message is a warning and doesn't seem to have a 3 digit error code. At least, it doesn't display it. Anyone know how to change a warning message in Apache? Janet