There is a particular kind of fragility that hides in plain sight in a well-run small business. Somewhere there is a spreadsheet, or a booking system, or a clever arrangement of folders and rules, that runs an important part of the operation, and exactly one person understands how it works. They built it, they maintain it, they know its quirks and its unwritten rules, and because they are competent and always there, the arrangement has never once failed. That very reliability is what hides the risk: a thing that has never gone wrong is a thing nobody has thought to protect.
The spreadsheet is the classic case because it is so easy to underestimate. It looks like a document, but over years it has become a program — with formulas that depend on other formulas, columns that must not be reordered, and a way of being used that is obvious to its author and opaque to everyone else. When that author is on holiday and something needs changing, the business discovers that it cannot safely touch the very thing it depends on, because a single wrong edit could break calculations nobody else can trace. The knowledge is not written anywhere; it lives in one head.
What makes this hard to fix is that the person concerned is rarely hoarding the knowledge on purpose. They simply never had reason to write it down, because they were always available to answer the question themselves. The gap only becomes visible in their absence, and by then it is an emergency rather than a task. It is the same shape as any single point of failure: cheap to address while everything is fine, expensive the moment it is not.
The remedy is not to replace the person but to make the knowledge survivable without them. Ask them to write a short plain-language note explaining what the thing does, what must never be changed, and how to do the two or three routine tasks it is used for. Have a second person actually follow that note once, while the author is still there to correct it — because a set of instructions nobody has tested is just another thing that lives in one head. The aim is modest and achievable: that the business could carry on for a fortnight if that person were suddenly unavailable, which is exactly the fortnight you cannot plan for.