People who know the outcome of an event tend to overestimate their own prior knowledge or others' naïve knowledge of it. This hindsight bias pervades cognition, lending the world an unwarranted air of inevitability. In four experiments, we showed how knowing the identities of words causes people to overestimate others' naïve ability to identify moderately to highly degraded spoken versions of those words. We also showed that this auditory hindsight bias occurs despite people's efforts to avoid it. We discuss our findings in the context of communication, in which speakers overestimate the clarity of their message and listeners overestimate their understanding of the message.