We have a 'this person better not be hit by a bus or quit on us' policy, or nearly so. We have people get assigned tasks from other projects from time to time,

We have a 'this person better not be hit by a bus or quit on us' policy, or nearly so. We have people get assigned tasks from other projects from time to time, and they start picking up bits and pieces of the projects from that. And we have an internal wiki that documents some things that people aren't very good about keeping up to date or making it comprehensive. Also some of our software has gone through rewrites that has cleaned up the architecture and made them easier to understand and work with, without documentation. But there's still way too much locked in people's heads. We've lost quite a bit of it, too, as people have left over time. I know we're trying to convince a client to shift over to newer versions of our software, because we no longer have anyone that knows anything about the tech the legacy legacy software was written in (it's legacy software that predates our existing legacy software).