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Diagnose a production outage caused by an expired TLS certificate, covering chain validation, intermediate vs leaf expiry, SNI-based multi-cert edge cases, OCSP stapling, and safe rotation without dropping connections.
## Problem
Your edge tier is fronted by six TLS terminators. At 03:47 UTC, PagerDuty fired on a 30% spike in 5xx errors scoped to one hostname, `checkout.example.com`. Browsers show a certificate error; internal service-to-service calls log `x509: certificate has expired or is not yet valid`. The other five listeners and most hostnames are serving normally. The checkout hostname runs long-lived WebSocket sessions that must not be dropped during rotation. Diagnose the failure, rotate the certificate without killing connections, and make this failure mode impossible to repeat quietly.
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