Different versions of your site can be running at the same time

It's pretty easy for a user to be running an old version of your site. Not only that, but a user could be running many different versions of your site at the same time, in different tabs, and that's kinda terrifying. For instance:A user opens your site.You deploy an update.The user opens one of your links in a new tab.You deploy an update.Now there are three versions of your site in-play. Two on the user's machine, and a third on your server.This can cause all sorts of exciting breakages that we don't often consider.