EMV fallback
When the terminal can't read the chip and swipes the magnetic stripe instead, falling back from the secure technology to the copyable one.
Chips get scratched, dirty or simply die, and a terminal that then refuses the card produces an angry customer. So EMV lets the terminal ask for a swipe after a few failed chip reads. The transaction is flagged as fallback, so the issuer can see it lacks the cryptogram the chip would otherwise have generated.
Fraudsters worked out what that meant almost immediately: a cloned stripe on a card with a deliberately ruined chip was a way around EMV altogether. The answer turned out to be financial rather than technical. Under the liability shift, which reached the US in October 2015, whoever runs the weaker technology eats the fraud, and most issuers now decline fallback outright or allow it only for small amounts. The fewer swipe readers left in the wild, the less of the problem is left too.