You hit the opener button on your way out and the door doesn't move the way it's supposed to. One side drops lower than the other, there's a metallic popping sound, or the panel just hangs there at an angle, wedged halfway up. You step closer and see it: a roller has jumped the track, or a cable has gone slack and is dangling instead of running taut along the drum. Your garage door has come off its track, and your first instinct might be to grab it and shove it back into place. Don't.