The following examples illustrate some of the many ways in which the implementation-dependent nature of the modification can manifest itself.
(let ((a (list 2 1 4 3 7 6 'five)))
(ignore-errors (sort a #'<))
a)
⇒ (1 2 3 4 6 7 FIVE)
OR⇒ (2 1 4 3 7 6 FIVE)
OR⇒ (2)
(prog foo ((a (list 1 2 3 4 5 6 7 8 9 10)))
(sort a #'(lambda (x y) (if (zerop (random 5)) (return-from foo a) (> x y)))))
⇒ (1 2 3 4 5 6 7 8 9 10)
OR⇒ (3 4 5 6 2 7 8 9 10 1)
OR⇒ (1 2 4 3)