Here's a nice pure function with predictable semantics:

#;6> (define (a foo) '(1 2 3))
#;7> (a 1)
(1 2 3)
#;8> (a 2)
(1 2 3)

But oh no you can break it:

#;9> (set-car! (a 1) 5)

And now it's broken:

#;10> (a 2)
(5 2 3)
#;11> (a 3)
(5 2 3)