How can interlocking directorates cause financial instability for universal banks? A detailed history of the Rotterdamsche Bankvereeninging in the 1920s answers this question in a case study. This large commercial bank adopted a new German-style universal banking business model from the early 1910s, sharing directors with the firms it financed as a means of controlling its interests. Then, in 1924, it required assistance from the Dutch state in order to survive a bank run brought on by public concerns over its close ties with Müller & Co., a trading conglomerate that suffered badly in the economic downturn of the early 1920s. Using a new narrative history combined with an interpretive model, this article shows how the interlocking directorates between the bank and this major client, and in particular the direction of influence of these interlocks, resulted in a conflict of interest that could not be easily overcome.