Byzantine Fault Tolerant (BFT) protocols rely on state machine replication to handle arbitrary behaviors. Significant efforts have been recently made to strengthen these protocols in order to minimize the performance degradation in presence of faulty components. In this paper, we focus on the potential damages that could be introduced from the client side of such protocols. In order to deal with this specific kind of threats, BFT protocols rely on request authentication to provide access control. Nevertheless, byzantine clients may benefit from the weakness of the underlying authentications mechanisms in order to tamper with the performance of replicated systems. We describe the main reliability issues that can be introduced by faulty clients in current BFT protocols, and we argue against the systematic usage of digital signatures. Finally, we propose a new policy in order to avoid the overhead due to systematic signatures verifications.