When someone with a lot of knowledge leaves a team, there is no way to extract it from them intact. (Telephone Game)
But we try anyway, and task departing experts with the drafting of documentation for when they've left (Writing is Hard), wasting the opportunity of an in-the-flesh expert who can help rebuild their knowledge in the next generation. (Knowledge is Personal)
And then that opportunity can be frittered away because we don't know the right questions to ask (Nothing Better than an Expert) because our awareness of each other's expertise has is filled with blindspots.