When I used to play rugby the rules wee a little different, two of the key differences between a free kick and penalty were:
A free kick can be charged down (as long as you start 10m back)
At a free kick the ball is live if it is picked up from the mark and then touched to the ground.
Knowing the rules, meant that on a couple of occasions after the opponents placed the ball down, I ran collected the ball and headed off down the field. At the very junior level I played at, maybe this was excusable a little.
I learnt the rule after seeing the same situation occur at an International, England against I think it was France.
It was a heat of the moment decision, not necessarily knowing or thinking about (if you knew) the intricacies of the rules is excusable to a degree.
I bet everyone takes this lesson to heart though.