Here we see an example of a chimney flue after a chimney fire. Fires can happen when creosote builds up from burning wood and not sweeping regularly, burning too much soft wood (example pine), or burning unseasoned wood (seasoned wood = cut, split, covered, and dried 3 years).
After a fire you may notice puffy or burnt creosote on your roof and even on the ground next to your house. Chimney fires can burn over 2000 degrees which often makes clay flue tiles crack from thermal shock as seen here. Thermal shock happens when the high heat of the fire meets cold air and cracks the tiles. Many people never know they have had a fire until it is inspected and others know right away by hearing the roaring fire or neighbors seeing giant flames coming out of the top the chimney.