Failure to observe properly is so easy to do. Many years ago when I worked in Bristol I was driving down the A46 and came up behind a slower car.
I had been followed by a dark blue Ford Sierra for the last 5 miles or so, and when an overtaking opportunity arose I indicated, checked my mirror, saw that the dark blue Sierra was still behind me, and pulled out - very nearly sideswiping another dark blue Sierra that was making the overtake too!
I hadn't spotted that there were two Sierras behind me, both the same colour.
