The actions below are usually perceived as a new page by users. And thus clicking the Back button should go back to the previous state.
 
  1. Opening an overlay & lightbox (the back button should close it)
  2. Filtering & sorting (the back button should go back to the previous filtering or sorting values)
  3. Going to the next step in an accordion checkout or a multi-step process on a page (such as a multi-step shipping estimator on the product page)
  4. AJAX pagination (Back button goes back to the previous page)
  5. Returning to the product list from the product page
  6. Expanding or truncating content (especially if the expanded content takes most of the screen)
  7. Clicking an anchor link (if they’re abruptly jumped down the page)
  8. Variations on the product page (depending on how many variations there are on average)