If the spring breaks and all the tension in them is released and it unwinds all of a sudden, the whipping metal wire will chop your skull in half and make a big mess.
Another thing people don't realise is if you are replacing or removing a garage door spring, don't hold it with your bare hand. The coils will collapse and pinch your skin in it, and you aren't strong enough to re-tension it (even if you're Mike Tyson).
Wrap a towel around the spring, and hold that. Then try and pull the towel off after, thinking about how much it would have hurt if it was your hand.