I got a real stack overflow by typing the following codes
(setf l1 (list 1))
(setf l2 (list 2))
(nconc l1 l2)
(nconc l2 l1)
Oops...
Is that a bug?
Not as such. Printing a never-ending list won't ever be a good thing.
Set *print-circle* to T to see the structure of the list.
It's no more a bug than the never-ending nature of #1=(foo . #1#)