Thermal asperity
A read-signal spike thrown when a magnetoresistive head grazes a bump — friction heat drowns out the magnetic signal.
A magnetoresistive head reads because its resistance changes with the magnetic field. But its resistance also changes with temperature, and that's the catch: brush a particle or a bump on the platter and friction heats the sensor within microseconds, and the resistance jump throws a large spike that swamps the tiny magnetic signal.
It appeared as a new failure mode in the mid-1990s with MR and GMR heads; the older inductive heads sensed no temperature and were immune. The read channel copes with detect-and-recover: recognise the spike, discard the sector and read it again.