FOD, Foreign Object Debris.
A former USN friend of mine was sent into the bowels of a tank in a ship with a small threaded piece which was found upon cleanup of said tank. His orders were to find out where it came from and to not come out until he put it back where it goes.
Of course he could not find where it goes. He was getting ready to drill and tap a hole in the tank which the threaded piece would fit (so he would not have to spend the rest of his tour in the tank) when he finally realized that he recognized the piece.
It was the screw which holds the adjuster in place on a Crescent wrench!
