London taxi drivers spend three or four years memorizing 25,000 streets. The output is a passenger delivered to Ealing. The byproduct is a measurably larger posterior hippocampus, growing in proportion to years driven. Bus drivers work the same traffic on fixed routes and don't develop it. Satellite navigation gets the passenger to the same door and leaves nothing behind.
Call it residue: what a process makes that nobody ordered and no one measures, because it was never the point. It surfaces years later, in a different room, as the ability to sense that something is wrong before you can say why.
Its absence is quiet. Nobody who already has the residue loses it; senior judgment stays intact, and that judgment keeps the place upright. The people who will never acquire it haven't arrived yet. So the reservoir doesn't drain, it stops refilling, and there is no quarterly line item where that would show up.
Deferred maintenance is real debt. Everything functions identically for a decade, and the bill arrives addressed to whoever is standing there when it comes due.
17% worse. In a 2026 randomized trial, junior developers learning an unfamiliar Python library with an AI assistant scored 17% lower on comprehension than those writing by hand — and finished only marginally faster.
The gap sat in debugging. Recognizing that code is wrong and working out why: the exact capability needed to supervise AI-written code.
Usage pattern mattered. Participants who asked conceptual questions scored well. Those who delegated the writing wholesale scored worst.
Desirable difficulties (Bjork, 1994). Practice conditions that slow you down today and produce more durable, transferable skill later. Struggle as the mechanism, not the cost.
Performance ≠ learning. How well someone does during training predicts poorly what they retain and can transfer.
Polanyi's paradox. "We know more than we can tell." Tacit skill accrues through doing, which is why it can't be reissued later as documentation.
The lag. Learning impaired now isn't tested until that cohort is asked for senior judgment, a decade out.