Purely hypothetical, of course...
Let's say you are hammering metal plates over studs to guard freshly-installed wiring in your basement. You are the only person in the basement. For the sake of argument, let's say that someone you live with, like your spouse or SO, comes down the stairs--which you don't hear because of your hammering--and starts talking to you when you have your back to them and are actively hammering. Surprised, on the very next hammer-stroke, you hit your thumb full-force, leaving a bad bruise, excruciating pain, and--what seems at the time--a potential break. (For clarity, lets say you have broken that very thumb before, so you have a good idea what it looks/feels like.)
Is it unreasonable to expect that this other person evaluate the situation and wait a moment or two before speaking?