Mine was the bow-versus-handshake moment in a work meeting — I went for a handshake, they'd already started bowing, and we ended up in this weird half-shake half-bow tangle in front of everyone. Nobody laughed at me exactly, but I definitely thought about it for days.
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?