Usage Based Reading (UBR) is a recent approach to object oriented software inspections. Like other Scenario Based Reading (SBR) techniques it proposes a prescriptive reading procedure. However, the impact of such procedures upon comprehension is not well known, and consideration has not been given to established software cognition theories. This paper describes a study examining software comprehension in UBR inspections. Participants traced the events of a UML sequence diagram through Java source code while thinking aloud. An electronic interface collected real-time data, allowing the identification of .points of interest ., which were categorised according to issues affecting participants' performance. Together with indicators of participants' cognitive processes, this suggests that adherence to UBR scenarios is non-trivial. While UBR can detect more critical defects, we argue that a re-think of its prescriptive nature, including the use of cognition support, is required before it can become a practical reading technique.