Counterfeit Penny is a Trinket in original Binding of Isaac and Rebirth. It's a shiny gold penny with sparkling.
Gives a 50% chance to gain +1 coin when picking up any type of coins. The +1 additional coin is automatically added to the total.
This trinket is not in the normal trinket pool and can only be obtained by destroying Golden Poops (33% chance).
This is not true in Rebirth, as it can also drop from Fortune Telling machines or other trinket generating constructs.