This example for APPLY is wrong, because it assumes particular behavior of TYPE-OF:
(defun foo (size &rest keys &key double &allow-other-keys)
   (let ((v (apply #'make-array size :allow-other-keys t keys)))
     (if double (concatenate (type-of v) v v) v)))
(foo 4 :initial-contents '(a b c d) :double t)
   =>  #(A B C D A B C D)
The call to FOO will signal an error if TYPE-OF returns a type containing the length (4). This causes a type conflict in CONCATENATE, because the new sequence length should be 8. If TYPE-OF returns a type without the length, then FOO will behave as shown above.

To get the intended effect, it could do something like one of these:

(concatenate `(vector ,(array-element-type v) *) v v)
(concatenate `(vector ,(array-element-type v) ,(* 2 size)) v v)