(defun foo (stream dispchar char)
  (declare (ignore dispchar char))
  (loop repeat 5 do (read-char stream) finally (return nil)))

(set-dispatch-macro-character #\# #\G 'foo)

(let ((*read-suppress* t))
  (read-from-string "(list #G1(((()")) ;=> error on CCL, NIL on SBCL

(let ((*read-suppress* t))
  (read-from-string "(list #H1(((()")) ;=> error on CCL, error on SBCL