https://github.com/shekhargulati/java8-the-missing-tutorial/blob/master/01-default-static-interface-methods.md

Rule 3: Otherwise, the class has to call the desired implementation unambiguously

Running with
openjdk version "1.8.0_144"
OpenJDK Runtime Environment (Zulu 8.23.0.3-macosx) (build 1.8.0_144-b01)
OpenJDK 64-Bit Server VM (Zulu 8.23.0.3-macosx) (build 25.144-b01, mixed mode)
will got error:
Error:(7, 8) java: class com.example.Test inherits unrelated defaults for test() from types com.example.TestAInterface and com.example.TestBInterface

public interface TestAInterface {
    default String test() {
        System.out.println("in a");
        return "a";
    }
}

public interface TestBInterface {
    default String test() {
        System.out.println("in b");
        return "b";
    }
}

public class Test implements TestAInterface, TestBInterface {
}

class App {
    public static void main(String[] args) {
        Test a = new Test();
        a.test();
    }
}