(setf *images* nil)
(dotimes (x 2) (setf *images* (append *images* (list `(:id ,x
                                                       :thumbs nil)))))
*images*
=> ((:ID 0 :THUMBS NIL) (:ID 1 :THUMBS NIL))

(setf (getf (first *images*) :thumbs) "a")

*images*
=> ((:ID 0 :THUMBS "a") (:ID 1 :THUMBS "a"))

Why is the :thumbs of every plist in *images* set to "a", instead of just the plist retrieved by (first *images*)? How do I get the latter behaviour? This caused a bug which took half an hour to identify...