Human errors provide valuable data for the study of the structure and the functioning of the human mind. Much research on memory is based on forgetting, work in perception is often based on discrimination failure, and the study of language and thought has exploited slips of the tongue. The study of human error is particularly important because (i) it demonstrates some limitations of the human mind, (ii) it sometimes reveals the rules or mechanisms that underlie people's behavior and (iii) it often suggests ways in which human performance can be improved.