This code is failing to compile:

import java.util.*;
import java.util.function.*;

class Test {
    public interface X {
        Object O = new Object() {
            Function<String, String> x(Map<String, String> m) {
                // Error: local variables referenced from a lambda expression must be final or effectively final
                return s -> m.get(s);
            }
        };
    }
}

reported by Paul Sandoz