When an error, any error occurs on save, ve.init.mw.Target#tryWithPreparedCacheKey assumes the error must be due to a bad cache key, and retries the request with the full (gzipped) HTML. Only when that fails does it pass the error to its caller. The problem with this is that there are many errors (such as badtoken, or CAPTCHAs, or anything else really) that cannot be remedied by retrying with the full HTML. tryWithPreparedCacheKey should only do this for actual badcachekey errors, not for all errors.