Opportunistic spectrum sharing has become a promising approach to fully utilize the scarce spectrum resources. In this paper, we analyze the performance of a wireless system consisting of two secondary user (SU) groups with different priorities, which are defined to satisfy the different quality-of-service (QoS) requirements. At the same time, we take the errors of spectrum sensing (i.e. miss-detection and false alarm) into consideration. We derive system performance metrics such as SU forced termination probability and blocking probability. Our results suggest the proposed model can improve the secondary system capacity significantly even under unreliable spectrum detection.