Common counterfeits of small copper shillings of King John Casimir, originally coined between the years 1659 and 1666, were researched reluctantly. The 17'hcentury hoard from Idźki-Wykno (Wysokie Mazowieckie County, Podlaskie Voivodeship), uncovered in 1969 gave us a new opportunity to take up this issue, thanks to its large representation of counterfeits.