The technical term for such errors is "split planchet". I personally don't consider them a true "mint error", since they usually do not manifest themselves until long after the coin has left the mint. And while they can occur naturally, anecdotal evidence says that it is all too easy to "create" one, simply by hurling enough perfectly normal-looking coins against a concrete wall or floor. Kids apparently did this with their pocketmoney back in predecimal days; if they got lucky, they'd wind up with two sixpences to spend instead of just one.
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