"I had gone away,—as it seemed, permanently,—but yet returned, like the bad half-penny"
The front and back of an 1851 half-cent
Public DomainThe front and back of an 1851 half-cent

The “bad half-penny” is one that has been minted with inferior metals along with the copper of which it was supposed to be constituted. Anyone who found themselves in possession of one of these counterfeits would pass it on to someone else at the first opportunity. As a result, they tended to circulate much faster than real coins, thereby increasing the chances of returning to the original owner’s purse.