The warning that appears everywhere can become part of the background it was meant to interrupt.

A warning that appears everywhere stops being a warning. Repetition converts it into scenery, and the signal that was supposed to interrupt attention becomes part of what attention filters out. The more universally something is posted, the less it can possibly mean.

The practical version of this is to spend your alerts carefully. If everything you flag is urgent, nothing you flag is, and this applies to how you raise concerns at work and at home just as much as to signage. And when you notice you have stopped seeing a warning, that is worth a second look, because the ones you have habituated to are exactly the ones that will catch you.