Do not make your disappointment the first notice of a rule someone missed.

Learning a rule through someone’s disappointment is a poor way to learn it. The person did not know, and the first information they receive now arrives wrapped in a judgment about them, which makes the correction harder to hear and the relationship slightly worse. An unstated expectation creates the fault it then punishes.

Before reacting, ask whether the rule was ever said out loud, and to this person in particular. If it was not, name it plainly now and let the incident count as the notice. Keep the disappointment, if any remains, for a second time.