By studying published information on various disasters throughout history, various patterns emerge. In studying these patterns we are able to deduce whether the failures were preventable or not. The methodology of this paper links all failures, as far as possible back to some human error, for example, design error, maintenance error, operator error, etc. Nine failures were analysed from the following industries: shipping, railways, airlines, nuclear power space exploration and chemical plant. Some failure sequences are simple, such as the Flixborough chemical plant explosion, while others are more complex, such as the Three Mile Island nuclear plant disaster. Of all possible types of error, avoidable operator error predominates.