Drilling and blasting is widely used in underground excavation projects, where the amount of damage to the surrounding rock mass is crucially important due to its impact on the safety of working environment and operational costs. The causes of overbreak are categorized into three groups namely: geological parametres, drilling accuracy and charging parametres. The present paper focuses on the special case of the Alborz Tunnel of Iran where a discontinuity surface located above the tunnel contour line caused excessive amounts of overbreak in the study area. After introducing the disconformity surface above the tunnel contour line, its impact on the occurrence of excessive amounts of overbreak is discussed. Possible case scenarios for future excavations are pointed outand the problems which may be encountered in each case scenario are predicted. Also, the impact of this special situation on the difficulties faced in working with rock mass classification systems is discussed.