The correct answer is counterfeiting.
Counterfeiting became a problem when the Ming government failed to issue enough copper coins.
The problem for the Ming government when they failed to issue enough copper coins was that the citizens still needed those coins to purchase things. So, there was not enough money to do so, and the people started to elaborate their own coins. They counterfeited the copper coins to have money to cover their needs, but the quality of the coin was poor.