As per regular on-line security practices you may receive a "Webpage has Expired" message if you attempt to navigate between pages using your browsers 'Back' tab or if a page remains idle for a set period of time.
We would always recommend that when navigating between pages on the Parkmobile site you use the respective 'Back', 'Next', 'Cancel', 'Submit', etc buttons in our forms, where displayed.
Please refer to your browsers support/help pages for full details.