1.) Lawn service guys direct their dust/grass clippings into traffic with leaf blowers, conveniently at your car as you drive past.
2.) The moron driving in front of you gets windshield washer fluid happy at anything over 40mph, even though you leave ~10 car lengths to prevent rock chips, the overspray still finds its way to your car.
3.) The random stray black cloud on a day that's forecasted to be perfectly sunny...and it just so happens to be floating over you and only you and needs to empty its bladder.
4.) The infamous hose running from the backyard swimming pool to the street emptying like 400 gal/min, creating a small river like environment that you must cross with your freshly detailing vehicle.
5.) birds. nuff said.
add to this if you want.