Our house is somewhere around 100 years old, give or take 20. The front door lock is original. Over time, the door frame itself has sagged, and the latch bolts no longer lined up quite right with the holes in the latch plate. This created a lot of resistance whenever you locked or unlocked the dead bolt, to the point where we had to lift the whole door upward (by the door knob) in order to (un)lock it. The brunt of this resistance was absorbed by the brass knob for the dead bolt, and it eventually broke. Since I'm easily distracted, we spent the next ten months (un)locking the front door with a screwdriver before I finally got around to repairing the lock. The fix was so simple, I should have done it right away.