Concurrent ArrayList Modification

Consider the following code. What is the most likely issue to arise when multiple threads concurrently modify the unsynchronized ArrayList?

import java.util.ArrayList;

public class EndlessAdd {
    static ArrayList<Integer> list = new ArrayList<>();
    public static void main(String[] args) {
        for (int i = 0; i < 50; i++) {
            new Thread(() -> {
                for (int j = 0; j < 1000; j++) {
                    list.add(j);
                }
            }).start();
        }
    }
}