Getting a page not displayed error when browser back button hit using php and session

I’m using sessions in my php code and am running into a very weird error.
As I’m going through my page flow and than decide to hit the back button on the browser, I get a "Page cannot be displayed error".

I really hate this because this means I have to add some funky code to make it work. The funny thing is is that if I put a back button in my php page and click it I get back to that page without any errors.

Can any browser guru’s help me out with this one??

Thanks