On a SERIOUS note we have had a few chimneys get blocked UP from the cleanout with dirt. If the liner is a single wall and goes below ground level thru a masonry chimney which loses its mortar it can rot thru and dirt and water can get in and block it up. Also a liner with holes can mess up the draft/downdraft on windy days and cause pressure switch problems.
Had a couple of rotted liners also 4 to 6 year old range. We determoned in both cases it was caused from the owners storing paint materials and such in the visinaty of the furnace. When mixed in the combustion process acid formes and rotted the liners. Both were residential homes. And both owners were the type that never through out anything. Now try getting them to belive this they still dident want to get rid of the paint etc.