Saw while editing https://www.mediawiki.org/wiki/Extension:TimedMediaHandler in VE in Chrome; while switching from visual to source editor, got an error popup generic "http" error. Looking at console, see:
Failed to load resource: the server responded with a status of 503 () https://www.mediawiki.org/w/api.php
After closing the in-VE popup about the error, the "Switching to source editing…" popup is still sitting there, with progress indicator moving along. Everything remains grayed out in the UI, and there's no way to recover my edits that I can see.
If a transitory error happens during the switchover, it should abort cleanly back to the view I was on before.