At least for Euro coins (are these more difficult to fake?) it often happened to me that some vending machine accepted a coin only after multiple tries.
What might be fun to try is to use a Euro 5c coin in a machine that expects 20p sterling coins. Living on the border with Northern Ireland, we figured out when I was young that the sweet machines in Northern shopping centres could be conned like that.
The old (pre ~2005) New Zealand 50cent coin was a match for 5 franc coin in Switzerland (~5USD). I knew postgrads that would always stock up to use in vending machines, after finding that out I've always been curious to know how much research a mint does to prevent this from being possible.
US 5 cent coins (aka nickels) are (or used to be) the same diameter and weight as 20p sterling coins as well, as a grad student in London I may have accidentally used a few in vending machines and doing laundry...
