I've noticed this while implementing a Rest-API today. We first define interfaces for each API method and the api contract for one endpoint allows (amongst others) two status codes: 200 and 204. Both may be returned when the request was successful, but the returned data isn't really important. The implementation might choose to return 204 and no additional data or a 200 if some additional, not really relevant data is returned (might be a string for debugging purposes like "entity 123 persisted").
Now, I'm implementing the API and know I want the implementation to always return such a text. But this might change in the future for whatever reason, so I thought I'll test the result like that (this is Java code but should be easy enough to follow):
assertThat(response.getStatus(), anyOf(is(200), is(204));
Is it acceptable to test my implementation like this, just because the interface allows both values for the same case (and it's not really important)? Or should I stay close to the actual implementation here (and change the test later, if I change the implementation)