Murphy's Law was codified on a submarine.

If you arm the warhead on a torpedo before you launch it, it will detonate before you launch it. (Murphy, op. cit.) So you put a device on the propeller shaft to count the revolutions, so that the torpedo warhead is armed only once the propeller rotates enough to move the torpedo a safe distance from the submarine that launched it.

But the engine on a torpedo can be started inadvertently while the torpedo is still in the torpedo room. Or, it can start unexpectedly when the muzzle door to the torpedo tube is still closed. (Murphy) So a separate impeller is installed on the side of the torpedo. When the torpedo moves through the water, the water flow turns the impeller. After a certain number of revolutions of the impeller, the warhead on the torpedo will be armed. Now the submarine that carries the torpedo around and launches it is safe, right?

Well, . . . no.

It seems that the berthing compartment on a submarine can only hold half the crew, and some crewmen sleep in the torpedo rooms. The crew is carefully split among the three different rooms in which they can sleep, so that all the sailors in one room can be killed, and still there is someone left alive with each of the skills that are necessary for successful operation of the submarine. Which means that a cook, a sonarman, a radioman, and a signalman may be among those sleeping in the torpedo room. Sleeping among the spare torpedoes. Very close to the spare torpedoes.

When the signalman is lying in his bunk, reading a book, or trying to fall asleep, there, above him, is an impeller sticking out of the side of a torpedo warhead. The impeller can be freely turned. You can reach up and flick it with your finger, and it will spin through a number of revolutions. And you can do it again. You can read several chapters of Louis L'Amour while idly spinning this peculiar impeller over your bunk. It is still there for your entertainment the next night.

Eventually, you can turn the impeller enough times, that you can arm the warhead on the torpedo. (Murphy) When the Fire Control Technician starts to set the torpedo for installation in the tube, his equipment tells him that the unit is fully armed. He is quite concerned, and he reports the incident. Similar incidents are reported dozens of times. A panel is convened to investigate the problem. The cause is discovered. Murphy's omnipotence is acknowledged yet again.