On a torsion-spring door, the failure is usually sudden: the door won't lift, or it drops hard and sits crooked in the opening with one cable gone slack. You'll often hear it before you see it - a loud bang from the garage, sometimes mistaken for something falling off a shelf. On older extension-spring setups, the sign is slower: the opener strains, grinds, or reverses itself halfway up because the spring has lost enough tension that the motor is doing all the lifting. Either way, the door gets heavy fast. A standard double door runs well over 100 pounds once the spring stops carrying its share, and an opener forced to lift that weight on its own will burn out a gear or a drive belt within a few cycles if it's not shut off.