Shared Array Modification in Threads

Consider the following Java code snippet. Which statement best describes a potential concurrency-related problem that could occur when this code is executed?

public class ConcurrentSum {
    private int[] numbers = {1, 2, 3, 4, 5};

    public void computeSum() {
        int sum = 0;
        for (int num : numbers) {
            sum += num;
            numbers[0] = 0; // modifying the array during iteration
        }
        System.out.println(sum);
    }
    
    public static void main(String[] args) {
        ConcurrentSum cs = new ConcurrentSum();
        Thread t1 = new Thread(() -> cs.computeSum());
        Thread t2 = new Thread(() -> cs.computeSum());
        t1.start();
        t2.start();
    }
}