Every product line has one. The engineer who knows that E-417 with the P2 pump variant is almost never the valve, it is the pressure-switch hysteresis, adjust it per §7.9 before you replace anything. It is in no document. They have explained it twenty times by phone. They retire in three years.
Why it is in no document
Writing a bulletin is work: reproduce the symptom, confirm the cause, write the procedure, get it reviewed, publish it. The engineer is too busy answering phone calls about it. The twentieth call is cheaper than the bulletin — for them, today. For the organisation, across three years and a retirement, it is the most expensive answer they will ever give.
Capturing it as a by-product
When an uncovered symptom escalates to the engineer and they resolve it, the resolution is recorded in the work order and the case. The third time the same symptom resolves the same way, the agent drafts a bulletin: symptom, root cause as the engineer stated it, procedure as they described it, the source work orders, the applicable models and revisions. The engineer edits — usually for ten minutes — and approves.
From that point the answer is in the index with their name on it. Technicians get it as a cited step. The question stops escalating. The engineer's phone gets quieter, which is the only incentive that works.
What the OEM gets
A documentation backlog that writes itself from the escalation report: the symptoms most often escalated are, by definition, where the manual is thin. And a knowledge base that grows from what actually happens in the field, cited to work orders, approved by the people who know — rather than from a technical-writing project that was budgeted for 2024.
What it does not replace
The engineer's judgement on the genuinely new. The point is to stop spending it on the genuinely repetitive.