Old one I’ve told before, not moving to a fixed hotend as soon as possible (v6 style hotends the mk3s uses for example, really easy to mess up) have absolutely accidentally loosed the block during a nozzle change, petg was leaking out of the top super slowly, only realised the issue once the blob started pushing the sock off.

Kept that hotend around for a while as a display piece, was a total write off. That also was the event that made me start keeping spares on hand, why I have my printers more or less part compatible for consumables (always keep a few extra heaters and rtds/thermistors around, do also have at least one extra hotend around).