When a Florida flight crashed in December of 1972, the black box was of immense help in determining the cause of the crash. Sadly, it wasn’t design error or mechanical failure – it was human error. The recording indicated that the cockpit crew became absorbed in changing an indicator light bulb. During this, the autopilot became disengaged, and they lost altitude. When the bulb was fixed, the crew wouldn’t believe their instruments (including audible warnings and alerts), spent precious time deciding whether to pull up or not, and eventually crashed.
Although tragic, from a design perspective there was not much more that could have been done to prevent this. There were quite a few alerts and procedures in place to prevent this kind of catastrophe. The story is however a cautionary tale for designers: it’s important to remember that when a person using your device or program is immersed in it, their focused attention might well drown out other things, including warnings and alerts. Therefore we need to always be aware of that possibility, and be prepared to repeat our message, depending on the seriousness of the situation.