Animal Nests in Chimneys
They love to build their nests and sleep in chimneys! If they do, the flue gases cannot escape through the chimney anymore and can enter the house through the fireplace which is a serious health hazard.
The picture shows an owl that fell into a chimney and was lucky to be saved. This can happen when chimneys don't have a chimney cap.
Bee and Wasp Nests
Bees and wasps often build their nests inside chimneys that aren't used regularly. These then form dangerous blockages in the chimney.
Racoons and Squirrels
Also like to breed in chimneys and often get trapped inside the chimney.