When ball is bounced from racket's top or bottom, not its side, it starts to tremble and moves incorrectly. OS: WinXP
Confirmed, the used algo is a bit naive, if the ball collide after passing the collision line but before getting out, it will collide multiple time with the pad, going a bit faster everytime, and get out randomly in any direction.
I'm going to close this as mostly not a bug (getting a good bounce is hard!), though I did add a note in the pong tutorial about it.