I got core with Steve's instruction on 21.2.5 on FreeBSD 2.2.7, with
or without window mode.
SL Baur <steve(a)xemacs.org> writes:
 Try this:
 
 xemacs-21.2.5 -nw -q
 C-x C-f ~/.emacs
 M-x font-lock-mode
 M-x fast-lock-mode 
Loading fast-lock.Fatal error: assertion failed, file bytecode.c, line 1904, OPAQUEP
(f->instructions)
Fatal error (6).
Your files have been auto-saved.
Use `M-x recover-session' to recover them.
	....
Lisp backtrace follows:
  fast-lock-mode(nil)
  # bind (command-debug-status)
  call-interactively(fast-lock-mode)
  command-execute(fast-lock-mode t)
  # bind (_execute_command_keys_ _execute_command_name_ prefix-arg)
  execute-extended-command(nil)
  # bind (command-debug-status)
  call-interactively(execute-extended-command)
  # (condition-case ... . error)
  # (catch top-level ...)
Abort (core dumped)