You'll usually see it before you feel it: a bowed or dented section low on the door, a seam where rust has started eating through the bottom edge, or a panel that's visibly out of plane with the rest of the door when it's down. Sometimes it's cosmetic and the door still runs fine on the track. Other times the dent has pushed the stile out of true, and the door binds or drags on that side as it travels. On older wood-composite doors, the symptom is different: swelling or soft spots near the bottom panel where moisture has worked into the material over years of freeze-thaw cycles. Either way, a damaged panel rarely stays a one-panel problem if it's left alone, since a bowed section puts uneven load on the rollers and hinges next to it.