The spec is missing "error_misc", but the example contains "error_misc".
When you label sbws fields "Exactly once." you are saying that they will always be in every line, in every future version.
If you want the flexibility to remove fields in later versions, please label them "Zero or one time.".
When you label sbws fields "Exactly once." you are saying that they have always been in every line, in every past version.
That's not true of the fields you just added, so please think about labeling them "Zero or one time.".
(Or you can just tell me that you're still developing sbws, and so you can break compatibility. That's ok, too.)