Assume that we are in initial round with a correct proposer (or with a proposer who has locked value in the highest round). Furthermore, assume that we are in GST period and that timeouts are large enough so communication among correct processes is reliable and timely. If a correct process p receives 2f+1 prevotes (any set of prevotes) before receiving Proposal he will prevote for nil. If faulty processes also prevote for nil (or stay silent), no correct process will precommit v, and we will not terminate in this round. This is not good as this is scenario in which we should always ensure termination.

Similar reasoning applies also for Precommit messages. If a correct process receives 2f+1 Precommit messages, before Polka and Proposal, he will Precommit nil, and again, we will not be able to terminate.