On the English Wikipedia article Hugh C. Robertson, I was trying to save an edit that I made with VisualEditor, but after filling in the edit summary and clicking "Publish changes", I got the following error:
I then clicked "Dismiss", closed the Save dialog, and tried to save the same edit again. Each time, it would give me the same 404 error.
The error came from https://en.wikipedia.org/w/api.php, with the following POST data:
action: visualeditoredit format: json formatversion: 2 campaign: summary: copyedit paction: save errorformat: html errorlang: en errorsuselocal: 1 page: Hugh_C._Robertson oldid: 915473825 basetimestamp: 20190913133954 starttimestamp: 20190913184637 etag: "915473825/f9cb9f80-d62b-11e9-8729-85d04a2e5aa3-df" html: [...] token: 68a45603c83eee982d1df6021d85452d5d7be40a \
The exact error data returned was:
{"errors":[{"code":"apierror-visualeditor-docserver-http","html":"HTTP 404","module":"visualeditoredit"}],"docref":"See https://en.wikipedia.org/w/api.php for API usage. Subscribe to the mediawiki-api-announce mailing list at <https://lists.wikimedia.org/mailman/listinfo/mediawiki-api-announce> for notice of API deprecations and breaking changes.","servedby":"mw1290"}After closing the window and starting from scratch, I wasn't able to reproduce the bug again.