When two people witness an event, they often discuss it. Because memory is not perfect, sometimes this discussion includes errors. One person's errors can become part of another person's account, and this proliferation of error can lead to miscarriages of justice. In this article, we describe the social and cognitive processes involved. Research shows how people combine information about their own memory with other people's memories based on factors such as confidence, perceived expertise, and the social cost of disagreeing with other people. We describe the implications of this research for eyewitness testimony.