Mine was keeping my shoes on indoors at a friend's place. Nobody said a word, but the pile of shoes by the door was screaming at me and I only noticed halfway through the evening. Now I clock the doormat situation before I even ring the bell.
Six weeks in and I greeted my neighbour with a cheerful wave instead of the proper hello, and got the coldest look of my life. Turns out there's a whole ritual I didn't know existed. What's the small etiquette thing you got wrong before someone finally explained it — the mistake you can only really learn by making?