The likelihood that when you select a locker (usually at gym) that someone will need to access the locker above, next to, or under the one you are using.
Gym Rat One: "Are you waiting for something?"
Gym Rat Two: "Yeah, my locker is below yours."
Gym Rat One: "What are the odds of that considering there are probably 100 in this locker room?"
Gym Rat One: "It's called Murphy's locker. Good workout?"