How much of your codebase is only understood by one person?
Have you seen this question before? If you're a programmer then you're likely to have. It's a question that is strongly related the concept of a bus factor; the minimum number of people that understand any part of a system. This question annoys me though, because by asking it you are assuming that the system in question is so complicated and ill documented that at least parts of it can only be understood by the people that created it or those who that knowledge was shared with.
Why is this the norm? Why is it that we accept complexity? Edsger Dijkstra famously said "Simplicity is prerequisite for reliability" and I don't anyone who would disagree. When we accept complexity by not striving for simplicity, we accept unreliability and bugs.
Well. Even well documented code is only understood by a small number of people at some point. Sadly life is never that simple.
I guess you also got to writing gist blog articles from HN? :D