Edits that would trigger this error silently break VE:
"Sorry! We could not process your edit due to a loss of session data."
On save, it shows animation indicating it is saving, but then does not save, leaving you in the editing screen. It doesn't refresh to update your session data, so the edit will never succeed; and it doesn't show you the error message.
To reproduce: start an edit, then logout, and try to complete; or v-v (and more common) start one as an anon, then log in.
Version: unspecified
Severity: normal