Unsafe acts, the drivers of operational risks, could be classified according to whether the act was intended or unintended. Operational risk of financial institutions especially arising from such unintended act or human error is increasing recently, but those were rarely examined in detail in the previous studies. In this article, authors try to identify proper strategies required to effectively manage such type of operational risk through case analysis. In general, there are two strategic approaches in managing the operational risk of financial institutions. The internal control approach, which is very traditional in the financial industry resulting in it being adopted by almost all the financial institutions, while the error management theory has been widely adopted in the process industries. Using the fund valuation agent as a case, this paper explores the characteristics of each approach and how such characteristics affect risk management performance. The finding proposes, when managing the operational risk arising from a mistake or an unintended act; firstly that an internal control approach emphasizing the segregation of duties and internal audit has a limited effect, and secondly that an error management theory focusing on the improvement of latent conditions, the design of an error-tolerent system and the installation of defence-in-depth is likely to have a better performance.