Maybe undefined behavior?

gistfile1.txt

(lisp-implementation-type)
"SBCL"
(lisp-implementation-version)
"1.3.9"


(defparameter a (make-hash-table :test 'equalp))

(defstruct b c d)

(defparameter b1 (make-b :c 0 :d 1))
(defparameter b2 (make-b :c 2 :d 1))
(defparameter b3 (make-b :c 0 :d 1))

(setf (gethash b1 a) 1)

(assert (eq (gethash b1 a) 1))
(assert (eq (gethash b2 a) nil))
(assert (eq (gethash b3 a) 1))

(print (list b1 b2 b3))

(setf (slot-value b1 'c) 2)

(print (multiple-value-list (gethash b1 a)))
(print (multiple-value-list (gethash b2 a)))
(print (multiple-value-list (gethash b3 a)))