Financial auditing, hereinafter referred to simply as auditing, plays a fundamental role in the economic decisions of stakeholders. Therefore, it is essential to ensure the quality of auditing by supervising the audit processes and audit firms by the Portuguese Securities Market Commission (CMVM). In this context, this study analyses the results of the CMVM's regular supervision of audit firms that audit public interest entities. To this end, a content analysis of the Global Results Reports of the Quality Control System of Auditing, issued by the CMVM, between 2018 and 2022 was conducted, with the aim of identifying the reported irregularities and understanding their evolution. The results revealed that most of the identified irregularities are related to audit files but are not significant. There was also a reduction in the irregularities identified over the period analysed, with 2018 and 2019 showing the highest percentages of significant irregularities. This study contributes to the identification of the irregularities pointed out and can be useful for both audit professionals and supervisory bodies in making improvements to their work.