Dented bottom panel
A dented or rusted-through bottom panel on one of these doors isn't always a simple swap-in from a big-box shelf. What you're usually looking at is one of two things: a single panel that's taken a hit from a car bumper or a plow, bowed or cracked, while the rest of the door still tracks fine - or a door where several panels have gone soft at the bottom from years of ground moisture, which is common on older concrete slabs that sit a little low.