When people say you need to be a vigilant driver, you really should take them seriously. While accidents aren't a guarantee, anything that does go wrong while operating a large, moving piece of machinery is instantly 10 times more dangerous than it would be otherwise. Even a moment of distraction can lead to disaster.
Unfortunately, though, being a cautious driver can only get you so far. There are just some circumstances that are truly too random for you to predict. Case in point: what happened to the poor driver in the video below. They're being careful and watching the road, but there is no way they ever expected this…
Just wait for it.
Oh man, I really hope they were up to date on their car insurance. Freak accidents like this scare the heck out of me.