Why is 3-D Secure / SCA failing on my Magento checkout?

3DS breaks when the redirect handshake doesn’t complete. Common causes: the return URL doesn’t match what’s registered with the gateway, the bank’s 3DS page is blocked from loading in an iframe by an X-Frame-Options / CSP header, or the Stripe / Adyen payment intent has gone stale before the customer finishes the OTP. Customers see Authentication failed or land back on an empty cart. We fix the redirect / iframe flow and re-test SCA end to end with a live 3DS test card.