A city library surveyed readers after replacing printed signs with a mobile map. The map, which the design team believed would shorten every search, helped visitors locate popular novels near the entrance. It did not, however, help many older visitors find local-history materials stored on another floor. The designers first blamed the size of the building, but interview notes revealed a different problem: the map used category names that regular staff understood but occasional visitors did not. The team then rewrote the labels, tested them with people who had never used the library, and kept the printed directory beside the elevator. Search time fell only after those changes. The case shows that a tool may appear efficient for familiar users while hiding a language barrier for others. A reliable evaluation therefore needs to separate the tool itself from the labels, assumptions, and users that shape its effect.