In this paper, error performance of the cooperative communication systems with the best relay selection scheme is investigated in the presence of error propagation from role to destination. The error propagation expression is derived firstly when the best relay is selected within M relays. The derived end-to-end BER expression is verified with the computer simulations. It is shown that the best relay selection does not ensure M+1 diversity order under the error propagation unlike perfect decoding. In addition, the threshold selection for the relays has dominant effect on the error performance of the system.