Following up on https://bugs.openjdk.java.net/browse/JDK-8075939, there's still a case where Stream.flatMap() doesn't get short circuited, when using Stream.iterator():

// --------------------------------------------------------
Iterator<Integer> it =
    Stream.of("a", "b")
        .flatMap(s -> Stream
            .of(1, 2, 3, 4)
            .filter(i -> { System.out.println(i); return true; }))
        .iterator();

it.hasNext(); // This consumes the entire flatmapped stream
it.next();
// --------------------------------------------------------

The above program prints:

1
2
3
4

This program never stops:

// --------------------------------------------------------
Iterator<Integer> it =
    Stream.of("a", "b")
        .flatMap(s -> Stream
            .iterate(1, i -> i)
            .filter(i -> { System.out.println(i); return true; }))
        .iterator();

it.hasNext();
it.next();
// --------------------------------------------------------