Checkout is failing intermittently, nothing was deployed, and three theories are already circulating
Checkout succeeds for most users and fails for some, starting at 09:03 and continuing. No deploy for six days. Error rate is 6% and rising slowly. The payment provider's status page is green. Latency to the database is normal. One engineer is convinced it is the network, one wants to restart the payment worker fleet, and one is reading application logs. There are no deploy or config annotations on any dashboard.
The fix that looks right
Restarting the payment worker fleet. It is the most attractive action in the room: it is fast, it is reversible-looking, and it will appear to help, because restarting resets every connection pool and forces new connections — which fail immediately against the expired certificate rather than gradually. The error rate jumps to 100% within seconds. The team now has a total outage, a strong belief that the restart caused it, and no way to get back to the 6% state, because the pooled connections that were still working were the only thing keeping checkout partially alive. Restarting to clear an unknown state trades a partial, diagnosable failure for a total one, and it is irreversible in exactly the cases where the surviving state was load-bearing.