D.Davidson argued that shared conventions learned in advance are not essential for the success of communication. In this paper, holding the validity of his contention in suspense, I argue that linguistic conventions play essential roles when communication fails. In everyday communication, when discrepancies are detected between what the speaker intended to inform the hearer and what the hearer actually understood, it becomes necessary to determine whether the speaker or the hearer caused the communication failure. For in everyday communication, the hearer often changes her position based on her misunderstanding about the intention of the speaker, and it is sometimes too late when the hearer realizes the speaker's true intention. In such cases, it is necessary to determine who is responsible for the hearer's loss. What the speaker said, which is determined by linguistic conventions, arbitrates the conflict between them. From the fact that shared conventions mainly relate to the evaluation aspect of communication, it follows that the speaker and the hearer need not learn shared linguistic conventions in advance of the conversation, and have only to learn them later, when failures are detected among past communication.