I almost got seriously broadsided the first time I encountered a St. Louis U-turn.

These happen at intersections with 4-way stops, which the city has in great abundance. The custom is to make a left-handed u-turn precisely in the center of the intersection, and signaling is not expected. If the car entering the intersection from your immediate right does not pause to make sure you don't turn, you can end up broadsiding them. U-turns are completely illegal on public roads where I learned to drive; I wasn't expecting something like that at all.