> Allegro CL 4.1 [SPARC; R1] (10/20/93 16:00)
> USER(1): '(setf . x)
> (SETF . X)
> USER(2): '((setf . x))
> Error: Attempt to take the car of X which is not a cons.
>   [condition type: SIMPLE-ERROR]

ACL 4.2 doesn't seem to have the anomalous behavior:

Allegro CL 4.2 [HP Prism; R1] (3/1/96 10:21)
Copyright (C) 1985-1993, Franz Inc., Berkeley, CA, USA.  All Rights Reserved.
;; Optimization settings: safety 3, space 1, speed 2, debug 3
;; For a complete description of all compiler switches given the current
;; optimization settings evaluate (EXPLAIN-COMPILER-SETTINGS).
USER(1): '(setf . x)
(SETF . X)
USER(2): '((setf . x))
((SETF . X))
USER(3): 


Wheeler
-- 
Wheeler Ruml, Aiken 220, <eecs.harvard.edu, at ruml> (617) 496-1066 (fax)
http://www.das.harvard.edu/users/students/Wheeler_Ruml/Wheeler_Ruml.html