Comment 1 for bug 1814367

(defclass foo () ())

(defun bar ()
  (break (make-instance 'foo)))

(setf (find-class 'foo) (defclass f () () (:metaclass sb-mop:funcallable-standard-class)))

(defmethod initialize-instance :after ((f f) &key)
  (sb-mop:set-funcallable-instance-function f (lambda (stream) (format stream "BREAK"))))

(bar)

=>

debugger invoked on a SIMPLE-CONDITION in thread
#<THREAD "main thread" RUNNING {10005F05B3}>:
  BREAK