I build a time machine and travel back in time to before the earth was lava.
The next user is attacked by a bloodthirsty wolverine.
I was on my bathroom break, so I emrged unharmed.
The next user is hit by a nuclear missile and is a odinary human out in the open without any ways of defence or escape.
Santa ruhes down the chimmy and yanks it out of my mouth, claiming it was his and you get arrested for ruining christmas.
Next user eats acid.
The acid was dilute acetic acid, also known as distilled white vinegar. While it tasted horrible, I am unharmed by it as it is not very corrosive to human flesh at such a low concentration.
The next user drinks concentrated hydrofluoric acid, you have no way of neutralizing it, and you die from fluoride poisoning before you realized that you drank the acid, since it killed all pain transmitters in the process of killing you.
luckily i spend my spare drinking acid anyway so i have developed an immunity to it's effects.
i poison the next user's water, put a bear trap on the seat that they sit down in to drink the water, place a gun under the table to go off when they sit down, hang a running chansaw from the ceiling that falls when the bear trap goes off, have a sniper across the road to shoot them as they drink the water, tell their neighbours that they support an ancient cult that eats babies, slaughter the next user's family, set their house on fire just before they sit down, and am waiting outside their front door with a detonator that is rigged to a small set of charges in the next user's ear. you must avoid all of that.
However, I break a small hole into the reactor wall, allowing the outside air to fill the vacuum inside the reactor, stopping the reactor from powering on.
I trap the next user inside the exploded reactor at the Chernobyl nuclear power plant.