It  does print something more useful in acl 5.0 (not yet released):



user(1): (defstruct (zzz (:conc-name nil)
		(:print-function print-zzz))
 (anything 23))
zzz
user(2): (defun print-zzz (obj stream level)
  (write-string "[ " stream)
  ;; next-lines to deliberatley throw an error
  (let ((element (car t)))
    (write element :stream stream))
  (write-string " ]" stream))
print-zzz
user(3): (setq test (make-zzz))
[ #<Printer Error, obj=#x20815242: Attempt to take the car of t which is not listp.>
user(4):