It generally appears on the POS body that has been used for a period of time, because the long-term use causes some debris like dust to cover the POS machine magnetic head, so when the card is swiped, the magnetic strip of the card body can not be in good contact with the magnetic head, resulting in a swiping error. For this reason, we recommend that you use a piece of hard paper with moderate hardness and gently drag it horizontally and vertically in the card reading slot to erase the dirt of the magnetic head.