There's no such thing as an "individual error" - every individual is operating within a management structure. Every error has the chance to be prevented before the fact or mitigated after the fact by proper quality management. Even if that individual is a lazy idiot who shouldn't be trusted with pointy scissors, whose fault is it for hiring him?

All errors exist on a spectrum of foreseeability and severity. Is it foreseeable that someone might miss a spot when cleaning? Yes, clearly, even if they're exceptionally diligent and well trained. The odds of them making that error diminish with training, skill and care, but they never reach zero. How severe are the likely consequences of that error? In this case, someone could die of sepsis.

You can't just shrug and say "the guy who was supposed to clean the device didn't do it properly, it's not our fault". Why did that error happen? What could management have done to reduce the probability of that error? How could that error have been detected before a patient was put at risk? From reading the story, I think it's abundantly clear that many opportunities to reduce risk are being missed due to poor management of the process.