Following is the list of all special variables with names without hyphens (as of Emacs 24.3.1). These should not be used as names of lexically scoped variables.
debugger features nil noninteractive obarray
The list is obtained with this code:
(require 'cl) (require 'cl-lib) (print (sort (let ((syms nil)) (mapatoms (lambda (s) (if (and (special-variable-p s) (not (string-match-p "-" (symbol-name s)))) (setq syms (cons s syms))))) syms) 'string<))
Instead of memorizing them, you can have Emacs highlight those names by putting the following code in your init file:
(eval-after-load 'lisp-mode '(progn (defconst my-special-variables-without-hyphen '(debugger features nil noninteractive obarray) "list of special variables with names without hyphen") (defconst my-regexp-special-variables-without-hyphen (eval `(rx bow (or ,@(mapcar #'symbol-name my-special-variables-without-hyphen)) eow))) (font-lock-add-keywords 'emacs-lisp-mode `((,my-regexp-special-variables-without-hyphen . font-lock-keyword-face)))))
The number of built-in special variables with simple names is decreasing on each release of Emacs. For example,
argi used to be special variables but no more.