What is the primary issue when multiple threads execute incrementCounter concurrently?

public class CounterArray {
    private int[] counters = new int[3];
    
    public void incrementCounter(int index) {
        counters[index] = counters[index] + 1;
    }
    
    public static void main(String[] args) {
        CounterArray ca = new CounterArray();
        Runnable task = () -> {
            for (int i = 0; i < 1000; i++) {
                ca.incrementCounter(1);
            }
        };
        Thread t1 = new Thread(task);
        Thread t2 = new Thread(task);
        t1.start();
        t2.start();
    }
}