Clojure doesn't give you immutability guarantees, it just makes it harder to chose otherwise, but on the other hand calling a Java method on some object is just one special form away. I'm not saying Clojure does the wrong thing here btw, but this thing you're talking about is a fallacy, unless you're working in Haskell and even there you could find ways to screw things up by interacting with the outside world, which isn't immutable.