Adjacent-track interference
Repeated writes to one track slowly erase its neighbours as tracks are packed tight — ATI.
The write field is never perfectly confined to the intended track; a weak fringe reaches over the neighbours. At loose track pitch it doesn't matter, but as track density is pushed up, enough writes to the same track will slowly degrade the data on either side. The effect is adjacent-track interference (ATI), a cousin of side erasure.
It is especially acute in shingled recording (SMR), where tracks deliberately overlap and are rewritten a band at a time. The remedies are refresh routines that rewrite exposed tracks in time, guard tracks between bands, and rewriting whole bands together rather than track by track.