Whenever i update my game this error appears.
the way to re-produce it is to update the game. open it and and let the new update files to be downloaded and then refresh the page and this error shows itself the way to solve it in most cases is to close all tabs and then open another tab to open the game because of this i suspect it has something to do with service workers. i've edited c2's sw.js file but i don't see edits making any problem you can see the edited version here : howoldilook.com/sw.js only added two event listeners one for push and another for notificationclick.
this problem occurs in chrome for both mac,android and ios.
sometimes the problem is more persistent and i need to empty the cache and localstorage which is a disaster for a game in production.