BetterTest.java

public void betterTest() throws Exception {
    // Do some setup
    // ...

    final Exchanger<Exception> exchanger = new Exchanger<Exception>();

    new Thread() {
        @Override
        public void run() {
            Exception thrown = null;
            try {
                int x = 0;
                // Check a few things that change x
                // ...
                assertTrue("An error", x > 3); // This fails!
            } catch (Exception e) {
                thrown = e;
            }
            try {
                exchanger.exchange(thrown);
            } catch (InterruptedException ignored) {}
        }
    }.start();

    // Wait for other thread
    Exception exception = exchanger.exchange(null);
    if (exception != null) {
        throw exception;
    }
}

BuggyTest.java

public void buggieTest() throws Exception {
    // Do some setup
    // ...

    new Thread() {
        @Override
        public void run() {
            int x = 0;
            // Check a few things that change x
            // ...
            assertTrue("An error", x > 3); // This fails!
        }
    }.start();

    // Wait for other thread
    Thread.sleep(2000);
}