The back button is an extremely important part of web navigation. Shoppers expect to be taken right back to where they were before engaging in whatever action necessitated the use of a back button when they press it. Implementing it might sound easy but things are rarely so simple. The problem comes when there is conflict between what you technically think is a new page and what a shopper considers to be a new page.

 

The actions below are seen by users as being a new page.

And thus clicking the Back button should take them back to their previous state: