LucidEmacs forked quite publicly from the FreeSoftwareFoundation’s implementation of Emacs in 1993. This was one of the more notorious code forks in the history of free software, probably because it was early and public. The Jargon file mentions it; the positions of some of the parties involved are available below. Past and continuing discussion on this Wiki of this subject is at EmacsSchismDiscussion.
Discussion on the Schism occurs at EmacsSchismDiscussion.