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.

>I've always been curious to know how much research a mint does to prevent this from being possible.

At the Canadian/US mint, seemingly zero.

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...

In the 80s I was able to use 5p coins (same size as the modern 10p) instead of quarters in arcade machines when I was on holiday in Florida.

The same trick worked with Australian $1 coins going into machines expecting NZ $2. imho having $1 larger than $2 isn't right.

Same for Euro notes. It often takes multiple tries of trying to straighten out the bill before a vending machine will eat it.