Figure the door weighs 200 pounds, and the spring can hoist its center of mass about five feet up. That’s 1000 lbf-ft of energy Now imagine all that energy launches a 1-pound chunk of broken steel spring; it’s as if that one-pound chunk has fallen 1000 feet (with no air drag), by which point it may hit speeds over 170 MPH. You generally don’t get all of the spring’s stored energy going into one ballistic fragment (ideally it’ll be a single break, and both parts of the spring will be safely retained on the spindle), but this helps to demonstrate what kind of mayhem is possible when a garage door spring breaks. A chunk of steel traveling at even a modest portion of that speed can maim or kill.
A few years ago I went into the garage and hit the button to open the door. It had just barely started to move when the spring broke, and the sound was like a fucking shotgun going off. My wife was elsewhere in the house and came running out to see if I was OK.