That's a fascinating question that really gets to the heart of what happens when we push automation to its limits. When the last human leaves a lights-out factory, it's not just about turning off the lights - it's about entering a realm where machines are completely on their own. Here's what I've found about those unexpected failure modes that only show up when nobody's around:

First, there's the cascade failure problem. In a fully automated system, one small glitch can trigger a domino effect. Maybe a sensor fails, causing a robot to misplace a component, which then jams the conveyor system, which then overloads the sorting mechanism. Without human eyes to spot the initial problem, these cascades can spread through the entire factory before anyone notices.

Then there's the 'alarm fatigue' paradox. Automated systems are designed to alert humans when something goes wrong, but what happens when there's no human to respond? The system might keep sending alerts into the void, eventually overwhelming its own communication systems or creating false patterns that confuse the AI's learning algorithms.

Another hidden risk is what I'd call 'silent degradation.' Machines can develop subtle problems that don't trigger immediate alarms - like a robot arm that's 0.5mm off its calibration, or a cooling system that's running 2 degrees warmer than optimal. Humans often notice these small deviations through intuition or experience, but automated systems might not flag them until they cause a major failure.

There's also the issue of 'unexpected interactions.' Automated systems are designed to handle predictable scenarios, but what happens when two perfectly functioning systems interact in an unexpected way? Maybe the vibration from one machine resonates with another, or the heat from a welding robot affects nearby sensors. These emergent problems only reveal themselves when humans aren't there to make on-the-spot adjustments.

Interestingly, research shows that unsupervised AI systems experience algorithmic failures at a rate 3.7 times higher than those with human supervision. The very act of removing human oversight seems to create vulnerabilities we didn't anticipate.

So when that last human leaves, the factory doesn't just run perfectly - it enters a state of fragile autonomy where small problems can become big disasters without anyone to intervene. It's a reminder that even the most advanced automation still needs some level of human oversight, even if it's just monitoring from a distance.