Bad Copper

The bad copper is usually found amongst other currency and appears as any other copper coin with the slight difference that the inscription around the outside is usually varied from what is common to the nation it is found in. 
This does mean that the coin will change appearance to fit in with whatever currency it is found amongst and will seem to change appearance when taken to countries that use different coins.  
Amongst the dwarf clans of Grimbordia is displays the words "Let the wealth of Grimbordian drown you." whereas similar coppers in the region have the words "Let the wealth of Grimbordian sustain you."

The elven kingdom of Sylfas normally stamps "A glint of sunlight" on their coins, the bad copper instead displays "A burning light". 

The coin currently displays "Minted by the fools of Param." instead of the normal "Minted by the United Peoples of Param".

The bad copper can not be gotten rid of by any means, with two exceptions. If used to pay for an item it will return to the owner's coin purse as soon as it is out of sight.  Where individuals have tried to keep the item from returning to its original owner by keeping it supervised, it has disappeared when they blinked.   If discarded it will likewise return.

If used to pay for goods or services there is a 5% chance that the person receiving the bad copper will notice its discrepancies and refuse to take it.

The owner of the bad copper will find a point during the day when they suffer from horrendous bad luck. This manifests in a similar effect to the bane spell.  
When the holder awakens at the start of the day the DM will roll a d20 in secret, this marks how many hours will pass before the bad luck hits.  Once that hour has begun, the holder of the coin will make all attack rolls and saving throws with a -d4 modifier.  Once the hour is over the user reverts to normal until the end of their next long rest. 

To remove the bad copper from a person it may either be targeted by a remove curse/wish spell or may be stolen by someone else.  If the coin is taken without the owners' permission it will attach to the thief instead.