I worked in a small place where this was a huge problem: There were only one or two software engineers working on the (multiple) products at any given time through the company's history. So when one person left, that could mean anywhere between 50-100% of the company's tribal knowledge of their own software is lost. Getting up to speed meant diving into the source code (if someone knew where it was) and stumbling upon old text file documentation basically by accident. Since they did not use source control (!!) there wasn't even a commit history to browse through to get even a clue as to in what state the products were in.