How a single missing switch-case silently misclassified agent sessions as healthy for hours

A software engineering team discovered a critical logic flaw in their agent-session health-check sweep, where an unhandled exit code (6, meaning rate-limited) caused the default branch to incorrectly label sessions as live. Because the default branch also reset a strike counter on every sweep, the misclassification was self-reinforcing, preventing any later sweep from correcting the error. One affected claim sat falsely marked as merely expired for over five hours, blocking 15 of 38 coordinated agents. A git history review revealed this was the third time the same default-branch trap had been patched — for exit codes 8 and 9 on prior occasions — each fix adding a named case rather than inverting the classifier's logic. The team acknowledged they had a documented engineering rule requiring exactly that inversion after a third such widening, yet failed to apply it in the same commit that triggered it.