It's known as «the bus problem». Peer review is used to address this, to make someone else a bit more familiar with your code, so he will be able to help you when you are overwhelmed and need help (or when you hit by a bus). Also, built-in documentation, run books, guides, and automated test cases are very helpful for those, who are not familiar with your code base. It reduces job security tough. (I hit by that multiple times, because, for management, it feels like good developers and good administrators are doing nothing or constantly pick-up very easy tasks to do).