You probably test a state machine by walking the paths you believe are legal: an order moves from pending to paid, then to shipped, and finally to delivered. That test passes and tells you very little, because the failures that actually freeze a service tend to come from the cells you never fill...
Source: [Dev.to](https://dev.to/codepy_1473/unhandled-events-not-broken-states-are-what-kill-your-state-machine-4dl6)