The tube that turned out not to be PTFE is a nasty surprise. On the fan side, if you run Klipper: heater_fan only switches the fan on above a temperature, it can’t tell that the fan is unplugged. A fan with a tachometer wire and tachometer_pin set at least shows you the RPM, and you could add a macro that pauses the print if it reads zero while the hotend is hot.

The low-tech version is to look at the heatsink fan spinning after any toolhead work, before the first heat-up. Cheap habit compared to rebuilding a melted toolhead.