Answer:  "Some people have particularly corrosive skin oils. I used to work in a machine shop, and we called those people, "rusters," and you never let them touch your tools. Rusters don't just rust steel, they also corrode brass, copper, bronze, aluminum, zink, etc. If a ruster used your parallels or square and you didn't clean and oil it right away, the rusted fingerprints etched into the steel would show up within a few hours. I'm guessing a ruster touched that penny at some point, after which is was left untouched for an extended period of time"