We had a cluster of servers, dynamically scaling up and down in response to load, and one day started seeing errors where an enum string field had an impossible value. Imagine the field is supposed to be "FOO" or "BAR" but one day in the logs you start seeing "FOO", "BAR", "GOO", and "CAR". Impossible. "GOO" and "CAR" did not exist in the code, nothing manipulated these strings, yet there they were.

Long story short, a particular machine that joined the cluster that morning had some kind of CPU or memory flaw that flipped a bit sometimes. Our Elixir server was fine because we were matching on valid values. Imagine a typed language compiler that makes assumptions about things that "can't" happen because the code says it can't... yet it does.