Concurrent Modification of an ArrayList

Consider the following code. When the main method is executed, two threads concurrently modify the myList object. What is the most likely issue to arise from this concurrent access?

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());
    }
}