You can follow your lockout/tagout procedures 100% but still have related dangers if you aren't actively looking for unsafe conditions. Any procedure that attempts to list such conditions would miss the vast majority of possibilities, even after reaching the point of being too bloated to be useful. Therefore, you must look at the environment you're working in, the equipment you're working on, the documentation you're working with, and anything else you might encounter before, during, and after locking out energy sources.
The problems aren't always obvious or directly related to what you're working on. Consider this example. A screw is missing from a termination box cover panel for 480V circuits. That panel isn't related to your work, it's just next to where you will be working. Why is this a problem? That panel effectively “locks out” the energy the panel contains. A missing screw reduces the protection. So, you haven't “locked out” all the energy sources that could harm you until you've replaced that screw.