A 2021 article by David Hall about shape coding:

Chapanis began interviewing pilots who had crashed B-17s and B-25s and a pattern emerged that turned his attention to the controls within the cockpit. As Fitts said ‘the intense effort to produce new weapons, the race against time in industrial production, and the magnitude of the program required to train men to operate these new machines resulted inevitably in many instances in which the final man-machine combination failed to function effectively.’

What Chapanis found when inspecting the cockpits of these planes were two identical toggle switches side by side, one for the landing gear, the other for the landing flaps. These controls were also similar in size and shape. […]

He modified the landing gear control by adding a wheel-shaped knob and a wedge like shape to the wing flap control. Now pilots could feel and easily map the shape to the intended purpose. […] Chapanis had solved a real life and death issue with one brilliant insight.

Chapanis was a contemporary of Fitts of Fitts’s Law fame.

I forgot this was called “shape coding,” or perhaps I never knew that? I have employed and sometimes pushed for a similar thing, but I called it making sure things have “distinct visual signature” or something like this. I think “shape coding” would be a more appropriate term.

The article shows one simple UX example – I would love to learn more about who’s employing this deliberately. It is, after all, the opposite force to consistency, and I’m always interested in negotiating with consistency.