Many reader/tag authentication protocols are proposed to effectively authenticate tags and readers. However, we demonstrate with YA-TRAP as an example how false authentications that a legitimate tag could be wrongly rejected by a reader may arise from these protocols when they are applied to C1G2 (class 1 generation 2) passive RFID tags. In this paper, we identify a protocol pattern of which the implementation on C1G2 passive tags leads to false authentications, and further identify three types of the existing protocols that can bring with false authentications due to containing this pattern. Moreover, we give a necessary and sufficient condition for false authentications prevention, and propose a semaphore-based solution which revises the pattern by adding semaphore operations so as to avoid false authentications. Our experiments demonstrate the arising of false authentications and verify the effectiveness of our solution.