After saving a new paste, the “View it live” link in the green alert banner opens in the same window. Once in that view, the back button doesn’t take you back to Pastebin but instead shows a “page expired” notice. One has to go back to the home page and then get back to Pastebin.
The “View Live” button at the bottom of the Pastebin screen opens in a new window so I suggest the one in the green alert banner work the same, for easier navigation.