I cannot believe, after all the waffle that's transpired in the last 3.89 years, that we haven't had a default thread of this nature. (Or maybe we have, I started it, and I really DO have dementia).
Anyway - my contribution. La Priorité à droite. We were caught out by this in Falaise, after riding hundreds of kilometres in France un-molested. We passed a crossroads that had the tell-tale sign (X) and a car approaching from the right clearly knew he had priority, even though we were hooning down a hill at 50. I learned (and have since forgotten, thankfully) some new words and hand signals.
Just watch the right folks! Even on roundabouts. They are mentalists.