The membership after the change.

The request was refused. The body is empty (Request body is empty.), not valid JSON (Could not decode request body.), or a JSON value such as 0, false or null; these get a string status and no title. A non-empty JSON value that is not an object gets the number 400 and the title Bad Request. An empty JSON list [] fails with 422 instead. Or the contact cannot be subscribed to this list: no email address for an email list, or no phone number for an SMS list.

The user may not change this membership. Nearly unreachable: the access check passes for any signed-in user.

No membership has this id, or its list was deleted. When the request does not set status 2, an unknown contact or list also returns 404.

The request was not valid: contactList, or its contact, list or status, is missing or not valid (one errors item per problem, with source.pointer naming the field). Refusing to resubscribe a contact who unsubscribed, when the user lacks the permission to resubscribe contacts, returns the plain sentence "Unsubscribed contact cannot be re-subscribed through API." instead of JSON, although the Content-Type is still application/json.