Steps to reproduce:
1) Add a lot of data to a site
2) Enable staging
3) Before the process finishes, go back to the site
4) You will get a [staging] [live] buttons in the dock
5) When you click on [staging] it will go to a Not Found page
After the staging process finishes, everything looks fine. There's nothing in the logs so it's hard to tell when the staging finishes.