@dan My favorite version of this is below.

The user's task, here, was to look at some data displays that were on the wall in front of these two controls, and then adjust one control or the other to regulate the operation of a machine.

The difficulty was that as built, the two controls were mechanically identical - it was hard to tell them apart by touch when you were looking at the data displays. The two controls had different enough functions that picking the wrong one was a bad idea.

The users attached beer-tap handles of different shapes to the controls, so they could tell them apart by feel. This allowed them to operate their *checks notes* nuclear reactor correctly.

(From "The psychology of everyday things", D. A. Norton, 1988.)