When inspecting up the smoke chamber, there should be a masonry joint where the smoke chamber transitions to the chimney flue liner. This joint should be parged smooth to prevent smoke from seeping between the flue liner and exterior brick. As smoke cools, creosote forms. Therefore, the smoke can seep into these holes and form creosote behind the flue tiles. If the chimney heats up during a chimney fire, then the creosote behind the tiles may reach an ignition temperature. This could cause a structural fire. Make sure this joint is in good condition.