114: Why does Emacs say "Key sequence XXX uses invalid prefix characters"?

  One of two things has usually happened.  In one case, the control
  character in the key sequence has been misspecified (e.g. "C-f" used
  instead of "\C-f" within a Lisp expression).  In the other case, a
  "prefix key" in the keystroke sequence you were trying to bind was
  already bound as a "complete key."  Historically, the "ESC [" prefix was
  usually the problem, in which case you should evaluate either of these
  forms before attempting to bind the key sequence:

                     (global-unset-key [?\e ?[])  ;;  or
                     (global-unset-key "\e[")