I've heard that when constructing fireplaces and chimneys, masons sometimes install a plate of glass partway up the flu. When the final installment on the work is paid, the mason drops a stone down the chimney to break the glass. Otherwise it's a house full of smoke for the deadbeat client...
I wonder what happens to the chimney sweep years later, the person that clambers down the chimney through unexpected shards of glass sticking out on the walls. Presumably they are aware of this as a possibility and take precautions.
Most residential chimneys nowadays are too small for people to climb into. Sweeping is done with a brush on a pole :)
Also, the glass pane would not have to be anchored deeply in the masonry, just set in slightly, enough to support its own weight. Thus any breakage would likely remove it entirely.
I've heard that when constructing fireplaces and chimneys, masons sometimes install a plate of glass partway up the flu. When the final installment on the work is paid, the mason drops a stone down the chimney to break the glass. Otherwise it's a house full of smoke for the deadbeat client...