Franz' ACL 5 has an interesting 'gotcha' related to backquote syntax.  Consider this form:

`(:a ,x :b ,y :c nil)

The list returned is not entirely fresh.  In fact, the last two conses comprise a constant list.  Therefore, if you destructively alter the last element, all lists generated from this form will be updated.  Ouch.  I am not sure whether this behaviour conforms to ANSI or not (I haven't looked).