The temperature of the heating element is regulated, but there can't be thermal sensors on everything. The typical disaster on 3D printers is that the print de

The temperature of the heating element is regulated, but there can't be thermal sensors on everything. The typical disaster on 3D printers is that the print detaches from the bed and sticks to the nozzle instead, which the printer doesn't notice, and so keeps extruding plastic into this obstruction, resulting in the nozzle being encased in a big blob of plastic. This isn't immediately a fire risk, because the hottest point will still be the nozzle which is temperature-regulated, but due to ongoing mechanical damage you start to risk things like the thermistor and heater wires being damaged, and that can cause havoc. Prusa recently introduced thermal modelling on their printers which detects heating anomalies like this, because the hotend becoming encased in plastic changes the rate of temperature leakage to the environment, and so the required heater power. The idea is that you can catch the issue at a very early stage.