People who carry unsecured or inadequately secured loads in the back of a truck or on a trailer or on top of a vehicle.
I have had numerous close calls over the years where I've had to react quickly to avoid disaster! For example once, while driving my Vett through Memphis on way home from Huntsville, a ladder came off the top of a van and I had to swing hard left to keep from having it come through my windshield. It was a reflex action and I was lucky there was no one to my left at that instant.
Another more recent incident occurred on I-40 near my house. One morning, on my way to work, a very large piece of plywood came off a trailer in front of me and came flying towards my car! Again, reflexes kicked in to avoided being hit. Luckily I am in the habit of not tailgating or I'd never have avoided it.
A good friend and a guy I used to work with lost his wife to a similar incident. Andy's wife was north-bound on I-25 between Colorado Springs and Denver when something came out of a truck in front of her. She reflexively jerked the wheel to avoid being hit. Unfortunately, her SUV rolled and she was killed. The thing that came off the truck was a large piece of styrofoam that would have done nothing had it hit her SUV.
Moral of the story: If you are hauling anything - SECURE YOUR LOAD!