Present technology is characterized by complexity, rapid change and growing size of technical systems. This has caused increasing concern with the human involvement in system safety. Analyses of the major accidents during recent decades have concluded that human errors on part of operators, designers or managers have played a major role. There are, however, several basic problems in analysis of accidents and identification of human error. This paper addresses the nature of causal explanations and the ambiguity of the rules applied for identification of the events to include in analysis and for termination of the search for `causes'. In addition, the concept of human error is analysed and its intimate relation with human adaptation and learning is discussed. It is concluded that identification of errors as a separate class of behaviour is becoming increasingly difficult in modern work environments. The influence of this change on the control of safety of large-scale industrial systems is discussed.