The picture below shows a brick chimney in obvious need of repair or re-building. The missing bricks collapsed back inside the chimney. Fortunately they made it all the way to the bottom of the chimney. If the bricks got stuck higher up in the chimney, the exhaust fumes from the heating and hot water system could have backed out into the basement. The combustion by-products / exhaust of both oil and gas fired heating and hot water appliances can contain carbon monoxide (CO) which is a deadly gas at high levels.

Although the condition of this chimney is quite apparent in this photo, this view is from the rear of the house, which from the ground was partially obscured by tree branches. The front side which was unobstructed, appeared to be in good condition.