Most finance teams have a person whose departure would be a real problem — not because they’re irreplaceable, but because a specific set of judgment calls and exceptions live only in their head, and nobody wrote them down because there was never a natural moment to.
A written procedure says "reconcile the bank account." It doesn’t say "this client’s payments always come in three days late and get flagged as a false positive — ignore it." That second kind of knowledge is where the real risk sits, and it’s almost never in the documented procedure.
Even a good handover document is accurate on the day it’s written and stale within a quarter, because nobody updates it as new exceptions get discovered — it’s a snapshot, not a living record.
The most common failure mode isn’t a mistake the replacement makes — it’s the weeks spent rediscovering exceptions the departed person already knew, one at a time, usually the hard way.
Waiting until someone’s leaving to document their knowledge means relying on their memory of what mattered, under time pressure. The better moment is the moment an exception is discovered — a decision recorded once, when it’s made.
This is close to literally how Train It Once works — capture the exception when it happens, not at offboarding. See Train It Once →