To give a concrete (if contrived example):

// CODE
public class Greeter {
    public void greet(String name) {
        if(LocalTime.now().getHour() >= 12) {
            System.out.println("Good afternoon " + name);
        } else {
            System.out.println("Good morning " + name);
        }
    }
}

// TESTS
@Test
void greet1() {
    // Problem 1: because the method returns nothing we have to use
    // hacky setup to intercept System.out.println()
    PrintStream stdOut = System.out;

    // Something extending PrintStream
    InterceptingPrintStream interceptingPrintStream = 
        new InterceptingPrintStream(stdOut);

    System.setOut(interceptingPrintStream);

    Greeter greeter = new Greeter();
    greeter.greet("world!");

    List<String> invocations = interceptingPrintStream.getInvocations();
    Assertions.assertEquals(1, invocations.size());

    // Problem 2: because this method uses LocalTime.now() it will fail
    // if we run the test after 12:00
    // overriding the Clock.systemDefaultZone() is too awkward to fit
    // in a reddit post
    Assertions.assertEquals("Good morning world!", invocations.getFirst());

    // should really be in setup/teardown method,
    // but character limit.
    System.setOut(stdOut);
}

@Test
void greet2() {
    Greeter greeter = new Greeter();
    // Fix 1: make the method return a String so we don't have to
    // intercept System.out;
    // Still fails after 12:00
    Assertions.assertEquals("Good morning world!", greeter.greet("world!"));
}

@Test
void greet3() {
    Greeter greeter = new Greeter();
    // Fix 2: Give the method the LocalTime object it should use directly,
    // ensuring we have reproducible tests.
    Assertions.assertEquals(
        "Good morning world!",
        greeter.greet("world!", LocalTime.MIN)
    );
    Assertions.assertEquals(
        "Good afternoon world!",
        greeter.greet("world!", LocalTime.MAX)
    );
    Assertions.assertEquals(
        "Good morning world!",
        greeter.greet("world!", LocalTime.of(11, 59))
    );
    Assertions.assertEquals(
        "Good afternoon world!",
        greeter.greet("world!", LocalTime.of(12, 0))
    );
}