(defclass foo-class (standard-class) ())

(defmethod validate-superclass ((c foo-class) (s standard-class)) t)

(defclass foo-object (standard-object) ())

(defmethod shared-initialize :around ((class foo-class) slot-names
                                      &rest rest
                                      &key direct-superclasses)
  (apply #'call-next-method
         class slot-names
         :direct-superclasses
         (append (remove (find-class 'standard-object)
                         direct-superclasses)
                 (list (find-class 'foo-object)))
         rest))

(defmethod update-instance-for-redefined-class :before
    ((instance foo-object)
     added-slots discarded-slots
     property-list
     &rest initargs)
  (error 'type-error :datum 2 :expected-type 'keyword))

(defclass foo () () (:metaclass foo-class))

(defparameter *foo* (make-instance 'foo))

(defclass foo ()
  ((slot :initform 42))
  (:metaclass foo-class))

;; the below results in TYPE-ERROR, as expected
(print (slot-value *foo* 'slot))
;; ...but after this is evaluated once and the ABORT restart is picked...

;; ...this results in UNBOUND-SLOT, *NOT* a TYPE-ERROR
(print (slot-value *foo* 'slot))