It seems that one of Joker's thugs at the beginning of The Dark Knight lost his consciousness because he was hit by something. However, there is nothing heavy enough to have caused such an effect in that scene (he got hit by a wood panel which weighed 5 kilograms tops). Is this a mistake in the film?
He got hit by the panel as you observed
BUT, it's not the weight of it that is important, it's the amount of kinetic energy that it carries.
A baseball doesn't weigh very much but at 90mph it will take you out if it hits you in the head.
A more extreme example is a bullet, weighs a couple of ounces but when fired from a gun it's lethal.
In this case, the panel (and I suspect it weighed considerably more than 5 kilos) has the force of a bus behind it and accelerates from zero to quite a speed before it hits the thug.