Sometimes as you are browsing the Web and hit the "Back" button, you get an error message telling you the previous page has expired.
Did the page actually expire in the twenty seconds since you were just there? It's not likely. The reason the error occurred is probably because the previous page was dependent on some sort of outside information you provided or was generated by some sort of script.
As an example, going "back" to a previous page when making an online transaction may cause you to accidentally order something twice. To prevent this from happening, a merchant's site may disallow you from going back to a prior page by showing the "Expired Error" message. Hitting the "Refresh" button, however, should allow you to re-post the information and reload the page.