Under classical mechanics you can in theory predict the state of any self-contained system, even the entire universe, if you know its state perfectly now.
Clearly quantum mechanics breaks that with the famous uncertainty principle. But in real life is there a good example where we can see divergent behaviour at the macro level e.g. can we see examples of a "quantum butterfly effect?"
I know the uncertainty principle is actually saying you can't measure the state absolutely in the first place but that's getting a bit deep for demonstrating an example of how the universe cannot be deterministic.
I'm after a layman's answer here, not a deep description of theory or wave-functions...