On the issue of failing for something that's not actually against the rules - surely that's possible in just about any skill test?
All skill tests will be assessing somebody's observance of what is considered (by the examiner) to be best practice.
A well known example in the UK is flight through a MATZ - we all know that to do so without permission is perfectly legal so long as you stay out of the ATZ itself. However, it is not very clever and even at PPL level you'd almost certainly fail for flying through a MATZ without achieving 2-way and obtaining a MATZ penetration clearance.
Similarly, Peter mentioned earlier that he'd been pulled up for not having his aircraft into wind for run-ups. This is not illegal, not downright dangerous, but most times, places and aircraft it would be regarded as best practice that you point an aeroplane into any significant wind for run-ups, and that if you don't, you do so deliberately and are able to explain and defend what you just did.
So in both cases, I suspect that Peter's "sin" may not have been doing what he did - but in not explicitly stating to the examiner "I'm doing things this way, here are my reasons for doing so."
G