Abstract: The design of software systems can exhibit several problems which can be either due to inefficient analysis and design during the initial construction of the software or more often, due to software ageing, where software quality degenerates over time. The design problems appear as "bad smells" at code or design level and the process of removing them is termed as Refactoring.. This paper presents the results from an empirical study that investigated the relationship between the bad smells and class error probability in three error-severity levels in an industrial-strength open source system. Our research, which was conducted in the context of the post-release system evolution process, showed that some Bad code smells were positively associated with the class error probability in the three error-severity levels. This finding supports the use of bad smells as a systematic method to identify and refactor problematic classes in this specific context.