A rule explained only after you break it has arrived too late to guide you.

A rule that appears at the moment of correction was never really guidance. It worked as a test you could not have studied for, and hearing it afterwards does nothing for the version of you who needed it. The gap belongs to whoever kept the expectation in their head. Being caught out that way is not evidence that you should have known.

If you set expectations for anyone, say them at the start and in plain words. And when an unwritten rule catches you, ask for the rest of them rather than accepting the suggestion that you should have guessed.