My wife and I rented a Citroen in Paris a few years back. All through France, Switzerland, and Italy people behind us kept their high-beams on and beeped their horns.

We knew the passing rules (stay out of the left lane unless passing), so we couldn't figure out why the other drivers were so angry at us. Did they hate Citroens? Did they know we were Americans?

No. It was the DAMN REAR FOG LIGHT! When you have the fog lights on, one very bright red light is illuminated in the left tail light assembly.

Once we figured that out at a gas station, we understood.