Very easily done, happens in an instant... I went into the back of a car about ten years ago when I was cycle commuting.
It was stopped in the road in a very unusual place on a long straight uphill stretch; in my defence I was cycling into a biting cold headwind full of large hailstones at the time, and had my head down (clearly a bit too much!) using my visor to shield my eyes.
I did spot it before I hit, but with the wet and my brakes not being particularly impressive anyway I didn't manage to scrub much momentum off before putting a decent dent in the boot lid! The driver was in the car at the time and took a fair amount of calming down, but I did of course pay for the damage...