As reported here:
http://mail.openjdk.java.net/pipermail/amber-dev/2018-October/003601.html

This code:
---
public class SwitchBug {

    static String hold(String item) {
        return switch(item) {
            case String s -> { System.out.println(s); }
            default -> "temp";
        };
    }

    public static void main(String[] args) {
        System.out.println(hold("bug"));
    }
}
---

compiles and produces:
---
bug
temp
---

This program should not compile, as the first case completes normally.