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).