Here's a slightly different bug I found before your update. It's another case where I don't know what behavior you expect, but it probably isn't this. :-p
(do (a <- '(1 2 3)) (lastcdr.a <- cdr.a) nil)
=> nil
(do (b <- '(1 2 3 2 3)) (lastcdr.b <- cdr.b) nil)
=> nil
(not+not+iso a b)
=> nil
(not+not+iso b a)
=> 1
(I'm using not+not+iso so it returns a predictable value rather than an address.)
