Great Renaming
The 1987 reorganisation of Usenet, which sorted the flat net.* groups into topic hierarchies like comp, sci and rec — and provoked the birth of alt.* in protest.
Before 1987 almost everything was called net.something, with a few mod.* groups for moderated discussion. With hundreds of groups that became unmanageable, and the informal club of major site administrators — the "backbone cabal" — pushed through a new order of seven hierarchies: comp, misc, news, rec, sci, soc and talk. New groups now needed a discussion and a vote. humanities was added in 1995, making the "Big 8".
When a proposal for rec.drugs was blocked, John Gilmore, Brian Reid and Gordon Moffett set up a parallel hierarchy in 1987 where anyone could create a group: alt.*. It soon outgrew everything else, which says most of what needs saying about how Usenet took to central planning.