The accuracy rate of frozen section constitutes an important step of quality assessment step in pathology practice. This study aimed to investigate pulmonary lesions that were incorrectly diagnosed or postponed for routine examination by pathologists at frozen section examination; it also aimed to discuss the reasons for difficult diagnoses and the various clues enabling the correct diagnosis to be made when such lesions are encountered. This study retrospectively reviewed the medical data of the thoracic surgery cases that underwent frozen section examination between 2009 and 2014. Frozen section errors and deferrals were identified in 25 cases. Fourteen (56%) lesions were of pulmonary parenchymal origin and 11 (44%) were of pleural origin. The number of cases in which the pathologists postponed the diagnosis without making any approach was 14. Of these, 9 (64%) were benign lesions such as bronchiectasis, fibrosis anthracosis, chronic inflammatory cell infiltration, chronic pleuritis, and mesothelial proliferation. The number of misdiagnosed cases was 11. Of these, 7 (64%) were of pulmonary and 4 (36%) were of pleural origin. Because the examination techniques of each pathology department may differ from one another, the comparative examination of frozen sections and routine sections would aid in becoming familiar with various pathologies and would be beneficial for pathologists in minimizing their diagnostic errors.