Serendipity in the library stacks is generally regarded as a positive occurrence. While acknowledging its benefits, this essay draws on research in library science, information systems, and other fields to argue that, in two important respects, this form of discovery can be usefully framed as a problem. To make this argument, the essay examines serendipity both as the outcome of a process situated within the information architecture of the stacks and as a user perception about that outcome.