People with punches, dies or other devices intended to stamp a design into metal frequently grab a coin from their pocket when they are looking for a piece of metal to test their device with. A coin is cheap (compared to going out and buying a piece of scrap metal, if you don't happen to have any just lying around) and when you've finished testing with it and provided you haven't mutilated it beyond recognition, you can still spend it.
Technically, it's against the law in Australia to vandalize coins like this, but unless the stamp bears some distinctive marking to identify the company or individual responsible, there's nothing the law can do and provided they're not mutilating a whole bunch of coins, there's no real motivation for them to try to hunt down the culprits.
Don't say "infinitely" when you mean "very"; otherwise, you'll have no word left when you want to talk about something really infinite. - C. S. Lewis