It might not be THE most, but was pretty amazing, so I remember it well.

I was driving to San Francisco, going over the Bay Bridge.

The traffic was kind of slow, not abnormally, but slow enough that groups of cars stayed in proximity long enough for me to observe this.

My attention was attracted to the action of the driver in a car in the lane next to me, as he flipped his still lit cigarette butt out his window.

Just as I was thinking “litterbug!” in disgust, the wind blew his cigarette butt back through the open window of the back seat of his car. He must have had some papers or something there, because they caught fire, so some minutes later the traffic got even slower behind his flaming car as he stopped it, and jumped out screaming.