By developing ideas which attempt to identify and explain some of the more commonly occurring psychological mechanisms of human error this article aims to provide a background of understanding which will be helpful to the systems analyst in his task of predictive reliability assessment. A simple hypothetical model of the mind is described from which human error mechanisms are explained. An underlying theme is that of communication. Accepting that accidents develop when there arises a persisting misalignment between the true state of a system and the operator's perception of that state, the article examines the processes by which this updating takes place. An experimental technique “communications analysis” is presented as a possible analyst‐aid for the prediction of error‐event sequences. The technique is illustrated by application to a nineteenth‐century railway signalling system.