The physical library shelves are regularly the site of serendipitous information discoveries, and are often sought out for this purpose. However, while some drawbacks to the shelves as information gateways have been documented, none to our knowledge relate to their capacity for facilitating serendipity. We present findings from a qualitative study of serendipity at the shelves, in which we identified a new drawback that we term the 'seeking-encountering tension'. On one hand, this tension entices people towards the relatively high-risk, high-reward activity of exploring new information avenues discovered serendipitously and, on the other, draws them back towards the relative safety of goal-directed information-seeking. We discuss the factors that contribute to this tension, and provide design suggestions for mitigating it. Understanding this tension can inform the design of physical and digital information environments that give users agency to switch between more and less focused information-seeking at will.