Emacs binds it in emacs-lisp-mode-map and nowhere else, so a mode derived from lisp-mode inherits nothing and the key answers "undefined" — which reads as the client being broken rather than as the key being free.
Emacs binds it in emacs-lisp-mode-map and nowhere else, so a mode derived from lisp-mode inherits nothing and the key answers "undefined" — which reads as the client being broken rather than as the key being free.