Painful retraction

["Bernard S. Greenberg" <BSG@SCRC-QUABBIN.ARPA>]

    Date: Thu, 23 Aug 1984  13:56 EDT
    From: "Scott E. Fahlman" <Fahlman@CMU-CS-C.ARPA>

    I don't think it would violate the standard for a particular
    implementation to return nil in this case -- this sort of extension in a
    situation that would otherwise be an error is generally allowed -- but I
    would strongly advocate that no implementor do this without a very good
    reason.  Users of his system will come to depend on this trick,
    sacrificing portability without realizing it.
I don't see any justification for such an extension.  Extend CDR to take
the successor of a number... how is that different?  NIL is clearly not
defined to be "a member of all data types" or similar.  I think the
proposed extension violates the spirit of Lisp.

What do you (Scott) think are similar situations "that would otherwise
be an error" that are really parallel?