Parsoid diff failure errors are not very descriptive - "error loading data from server: unsuccessful request: diff failed." but not why (in this case, it was because the target oldid had been deleted subsequent to the user pressing save, but a more graceful message would be good.
Version: unspecified
Severity: enhancement
See Also:
https://bugzilla.wikimedia.org/show_bug.cgi?id=44354
https://bugzilla.wikimedia.org/show_bug.cgi?id=39057
https://bugzilla.wikimedia.org/show_bug.cgi?id=63149