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This letter introduces an observed spectrum sensing issue that: with certain probability, a secondary user (SU) misinterprets a non-interfered primary user (PU) is being interfered, resulting in access opportunity loss. This issue termed as spatial false alarm (SFA) problem is one of determinants on SU's medium access probability. However, it is neglected in current research related to spectrum sensing. By thoroughly study, the principle of SFA is presented to reveal the cause of this problem. Moreover, we quantify the impact of SFA by a closed-form expression to correctly evaluate the medium access probability for SU. The simulation results evidence both the importance of SFA and the validity of the derived medium access probability.