- The feature was not documented fully because 'the team doesn't do that sort of thing'
- The feature was not documented because 'it is so minor, it's not worth documenting'
- The feature was implemented by someone that needs (ahem) oversight.
- First, Stop Running Your Planned Tests. Really, you aren't going to get anywhere.
- Start exploring what was implemented. You may find that it's your misunderstanding and not the developers.
- Keep good notes.
- Raise a Red Flag. Either you need to re-write your tests or the software needs to be re-written. Either way, this is going to affect the schedule.
- Re-test once this is cleared up.