Mastery is when the code you wrote 6 months ago, or a year ago, or five years ago is still running, and not causing problems. Mastery is when the code you write brings long term benefits, and not future problems.

I've both seen and written code like that, so it does exist. Like anyone one else I've also written code that causes future problems. I believe I'm at a point in my career (25 years in) where I now write more of the former than the latter.