This reminds me of a story I read once about how Bell Labs engineers wanted to find the ideal length of a landline telephone chord. They did this by secretly shortening the chord over time on test users' telephones and waited to see how short they could make the chord before they noticed. If the plugtest is hardware-hardware testing Bell Labs engineers were doing human-hardware testing.

I tried to find a source for the story but I couldn't find one. I think I read it in The Innovators by Walter Isaacson, but I can't remember it exactly - maybe I'm misremembering it.