Hi,
Our editors have made several reports of being unable to save an item (genericError MISSING_FIELDS). After a bit of investigation it turned out that they receive this error if they have their browser tab open for quite some time, and new fields are added to the data schema by our admin in the meantime.
A more elegant approach would be to show the new field(s), and provide the editor with a more human friendly error message. Or ignore this when this happens with non-required fields.