If the accept header of the request doesn't belong to the resource, it will return the data anyway.
I've seen no mention to what should be the expected behaviour when the wrong parameter is passed to accept header.
( https://github.com/ezsystems/ezpublish-kernel/blob/master/doc/specifications/rest/REST-API-V2.rst )

For example: Get content type (article)

Header should be (xml or json):

GET /api/ezp/v2/content/types/16
Accept: application/vnd.ez.api.ContentType+xml
Authorization: Basic YWRtaW46cHVibGlzaA==

However it will work if it is:

GET /api/ezp/v2/content/types/16
Accept: non-existing-parameter
Authorization: Basic YWRtaW46cHVibGlzaA==

Or even:

GET /api/ezp/v2/content/types/16
Accept:
Authorization: Basic YWRtaW46cHVibGlzaA==