In this paper, non-orthogonal multiple access (NOMA) with successive interference cancellation (SIC) technique for downlink channel of next generation wireless network and its security issues are discussed. In NOMA, signals for all user equipment (UEs) are superimposed before transmission and then decoded at each UE implementing SIC. SIC relies on decoding and subtracting the signals in sequence until it reaches its desired signal. The decoding order at each UE, however, must match with the user index in the cancellation sequence. In case of mismatch, UEs will not obtain their signals and re-request their messages from the BS which will further increase the processing time that is already high for a regular SIC receiver. Numerical results show the performance of SIC at the UE side. The time spent for decoding the received signal at the UE with and without user mismatch is discussed. The unusual longer decoding times are further discussed as potential external network threat.