Head crash
When the read/write head touches the spinning platter and scrapes away magnetic coating and data in a fraction of a revolution.
A head flying a few nanometres above a platter at thousands of rpm has no margin. A knock, a dust particle, a worn bearing or an abrupt power cut can let the head strike the surface; frictional heat and the loosened coating destroy the tracks beneath it, and the debris torn loose causes fresh crashes in a chain reaction. The sound — a repeated ticking — earned the phenomenon the nickname "click of death".
The risk of a head crash is exactly why hard drives are hermetically sealed and why the heads are parked outside the data zone when the drive powers down. A crash is usually irreversible: unlike a logical deletion, the physical medium is gone, and recovery demands a clean room and replacement heads.