A room’s customs become obvious only after a newcomer accidentally violates one.

Rules that everybody follows stop being visible to the people following them. Nobody could list a household’s habits until somebody sits in the wrong chair or opens a cupboard that is never opened, and then the whole invisible system becomes briefly obvious. The newcomer has not done anything wrong. They have run into something nobody wrote down. Embarrassment on their side and surprise on yours arrive together.

When a visitor trips one of these, say what the custom is and laugh at the fact it was never mentioned. Somewhere new yourself, ask early rather than deducing the rules from behaviour.