Agreed, that sort of prank is an act of gross stupidity.
f**king morons 
Yea, fkin idiots!
There is something similar but much worse in many Brazilian cities.
Kids in some areas like to play with simple wood and paper kites which are flown very high and are sometimes used to fight. In order to cut the line of another kite, a mixture of wood glue and crushed glass called ‘cerol’ is applied to the aggressor’s line; then, when your line crosses that of another kite, all you do is yank it up and down making a sawing motion and the other guys line will snap!
Of course, he probably had cerol on his kite line, so when his kite falls to the ground the line can get caught across roads, strung over walls etc.
This piece of text describes the danger of cerol -
" * Quais os riscos do cerol para motociclistas?
Devido a falta de proteção do pescoço do motociclista este é o mais atingido. Neste local passa uma artéria de grande calibre e o corte desta pode provocar um sangramento muito intenso causando a morte em poucos minutos. Existem casos de pessoas que tentaram retirar a linha do pescoço e tiveram seus dedos amputados. Já fiquei sabendo de um caso no Rio de Janeiro que a linha atingiu os olhos do motoboy e ele perdeu uma visão, este motoboy não usava capacete e nem óculos de proteção* ."
I’ll attempt to translate -
“What are the dangers of cerol to motorcyclists?
A lack of neck protection has to be the worst. A very large artery is in this area and a cut would produce intense bleeding causing death within minutes. There have been cases when people have tried to remove their helmets and had their fingers amputated. We already know of a case in Rio de Janeiro where line (with cerol) reached the eyes of a motoboy (motorbike courier) and he lost his sight, this motoboy wasn’t using a helmet or goggles.”
This video gives you an idea even if you don’t speak Portuguese.
http://www.youtube.com/watch?v=CAv-0SzbIEI
This one is quite graphic, although there is no blood - watch?v=y6eymI2CLjk (add that text after the YouTube URL)
Absolute w*nkers!
A friends 8 year old son got badly hurt when cycling over a local park in very similar circumstances, but I don’t think it was deliberate.
The tennis players had broken the net mount so pulled the line through the wire fence and tied it round a tree outside the court. My friends son was cycling between the court and the tree, didn’t see the cord and got hit precisely in his neck.
The injuries were bad and took weeks to heal BUT he was only pedaling at a very slow speed – I dread to think of the damage this type of thing could cause at speed.