People who do things a lot make mistakes, and they usually end up making the kinds of mistakes a rookie wouldn't, because a rookie tends to be more cautious and thorough. Once someone gets something down to muscle memory it's easy to have a lapse and skip a step and move right onto the next without even noticing.

That saying 'a gun is always loaded' is kind of a no-brainer, but it's mostly beaten into the heads of people who are constantly around guns because they're the people who are more likely to have a brain fart and incorrectly assume the gun they just cleaned is not loaded. That type of deal. It's so common sense that it goes without saying, and that's why it's said over and over again.