Control systems for airliners, military aircraft, automobiles, and for the safety of nuclear power plants are typical of the critical digital systems addressed in this paper. These systems are considered safe by the public: their accident rate is sufficiently low that it does not prevent their widespread acceptance. Nevertheless, developers, regulators and users would like to see further improvements in dependability. Accidents of scheduled air carriers are very rare, but when they do occur they are exhaustively investigated. The public record of these investigations is therefore a good starting point for exploring dependability improvement in critical systems. Examples presented in this paper show how current development practices permitted hazardous situations to exist and a methodology for reducing the frequency of such hazards is presented.