A trivial illustration is putting money into a vending machine, obtaining the desired item, but failing to receive change. Each action is appropriate in and of itself, but giving no change is not appropriate given the sequence of actions and states before it. When the machine is a computer standing by itself, then these misbehaviors can mostly be traced to their origins in the design of the system, software or hardware. But suppose this system is not isolated, and communicates with other such systems, as well as human operators, in a changeable environment. In an aircraft, say.
Failure analysis becomes technically complex and intellectually difficult.
Aircraft accidents are amongst the most carefully researched failures in all of engineering. One would expect that the reports are exemplary. Simple logic mistakes appear to have been made. We believed that there must be a rigorous method of reasoning that would allow objective evaluation of events and states as causal factors. We thus developed the WB-graph method.