When multiple threads run ListUpdater concurrently, what is the most likely issue that may occur with the ArrayList?

import java.util.ArrayList;
import java.util.List;

public class ListUpdater implements Runnable {
    private List<Integer> list;
    
    public ListUpdater(List<Integer> list) {
        this.list = list;
    }
    
    public void run() {
        for (int i = 0; i < 1000; i++) {
            list.add(i);
        }
    }
}

public class ListTest {
    public static void main(String[] args) throws InterruptedException {
        List<Integer> myList = new ArrayList<>();
        Thread t1 = new Thread(new ListUpdater(myList));
        Thread t2 = new Thread(new ListUpdater(myList));
        t1.start();
        t2.start();
        t1.join();
        t2.join();
        System.out.println(myList.size());
    }
}