ArrayList Thread Synchronization Issue

In the following code, a writer thread synchronizes its additions to a shared ArrayList. What concurrency issue may still occur due to the reader thread’s unsynchronized iteration?

import java.util.ArrayList;

public class SynchronizedIterationTest {
    static ArrayList<Integer> list = new ArrayList<>();
    public static void main(String[] args) {
        Thread writer = new Thread(() -> {
            synchronized(list) {
                for (int i = 0; i < 50; i++) {
                    list.add(i);
                    try { Thread.sleep(10); } catch (InterruptedException e) {}
                }
            }
        });
        Thread reader = new Thread(() -> {
            for (Integer num : list) {
                System.out.println(num);
                try { Thread.sleep(15); } catch (InterruptedException e) {}
            }
        });
        writer.start();
        reader.start();
    }
}