The handover problem.
Most handovers only capture a fraction of what the employee actually knows.
When someone leaves, the official handover usually covers tasks, passwords, documents and a few loose notes. The real knowledge is rarely written down. It sits in old emails, spreadsheet logic, client preferences, supplier relationships, internal decisions, shortcuts, exceptions and years of context built up inside one person's head.
When that person leaves, the business pays twice. Once when the employee learned the role. Again when the replacement has to rediscover it.

