Hidden in plain sight
I work in the field of machinery automation and I remember visiting a customer in Italy to conduct an upgrade of some sort. The machine compiled a production report that it spat out on a dot matrix printer at the end of every shift. I guess the original engineer who commissioned the machine some years previously had a falling out with the production manger as ever 8 hours it had been printing out a "W@nker Report"
Comments in code can be hard to kill off too! The common unit of un-quantifiable things around here is the banana. This stems from a comment I wrote in a piece of "C" code many years ago while struggling to commission a machine in the jungles of Indonesia. I had modified an inertia calculation and added the immortal comment: "Add a banana for good luck!