I really hate those people changing lane without indicating first. In fact, this is a very stupid move as other drivers on the road can't read your mind as you move out of the origional lane suddenly. ...
Btw,when I was learning driving back in Australia. I was taught to indicate and turn your head to check for other traffic before changing lane. (Don't know what the practice is in HK?) It is because there is a spot called the BLIND SPOT which you can't spot the car in the other lane on your mirror.