"Rappers" on the roof of the electrostatic precipitator knock the accumulated dust free, letting it fall into the storage hopper. Each rapper is the size and shape of a baseball bat. Inside is an electromagnet that pulls a steel plunger upward, then allows it to fall again, producing a sharp knock. The rappers are energized at seemingly random intervals, producing a haunting, syncopated music. (The rhythm seemed more modern jazz than rap.)
Today population forecasts are based on extensive and reliable data. However, no such demographic base exists for the world's growing population of machines and devices. Now may be the time to take machine demography seriously and enter into real discussions about machine population control.
The couple of years in question here saw one of the largest bureaucracies anywhere undergo a convulsion in which it tried to reconceive itself as a non- or even anti-bureaucracy, which at first might sound like nothing more than an amusing bit of bureaucratic folly. In fact, it was frightening; it was a little like watching an enormous machine come to consciousness and start trying to think and feel like a real human.
Perceptual: "They couldn't figure out what to do next", "they couldn't find the feature", "they didn't know they could click that button..." etc.
Domain-specific: "We need a way to jump back here because in their workflow this happens..."
In general, usability testing only catches type 1 perceptual problems. Because in those tests you take people out of the real world and assign them tasks. Usability testing doesn't catch domain-specific problems because they only come up in real life use.