I’m a user experience designer. My favourite story is from aviation engineering. I don’t remember the year or all the details, but the US Navy had put stupid amounts of money and time into engineering a new fighter jet. It was worked out on paper and built to exact specifications. Then, during the first human test of it, the pilot ejected on the tarmac before it took off. The plane crashed, obviously, but the pilot couldn’t explain what happened (apparently he had a concussion from his unscheduled landing).

The plane was built again, and shortly after takeoff, the pilot again ejected without explanation.

What the fuck was going on?

In the retelling I heard, someone finally noticed the design of the cockpit was to blame. In trying to cram all the standard controls plus new ones into the smallest amount of space, the designers had moved the eject lever right next to the lever to adjust the seat position – they’d coloured the eject lever red, but the pilot couldn’t see that since it was below and slightly to the right of his ass, and both levers were the same size and shape. Nobody noticed this was a problem until at least two pilots accidentally ejected on takeoff.

This might be apocryphal, I don’t know, but I learnt it as an example of how things might look good on paper, but you can’t really know until a user fucks everything up.