Greece has a habit of bringing down Monetary Unions

This article covers the brief history of Monetary Unions, apparently the earliest attempt was recorded in 400 BC between the Greek city-states. This features different Monetary Unions, but I found the Latin Monetary Union interesting. It was a union that was started 1866 with France, Belgium, Italy, and Switzerland. Now remember, the domestic gold standard was still a policy within these countries. Instead of sharing one legal tender, the currencies were pegged to each other.

Once Greece gained it's independence from the Ottoman Empire, it's fragile economy joined the union. Needless to say, they didn't contribute much to the union and the union eventually failed with the beginning of World War I.