Oops! Your theme editing session has already expired and no changes have been saved

The error code is just a session expiration.

19 hours ago, David N. said:

Got it, thanks. And I haven't seen it again so maybe I had left that theme editor open for too long, that's probably it. 

I'll make a note to see if we can be more graceful about that, but it just looked like the user session timed out that stores the flags that a theme editing session is in progress.