Analyzing An ArrayList Merge Method

The mergeSort and merge methods below are used to sort an ArrayList of integers. Which statement describes a potential pitfall of the merge method’s implementation?

public static void mergeSort(ArrayList<Integer> list) {
    if (list.size() <= 1) return;
    int mid = list.size() / 2;
    ArrayList<Integer> left = new ArrayList<Integer>(list.subList(0, mid));
    ArrayList<Integer> right = new ArrayList<Integer>(list.subList(mid, list.size()));
    mergeSort(left);
    mergeSort(right);
    merge(list, left, right);
}

public static void merge(ArrayList<Integer> list, ArrayList<Integer> left, ArrayList<Integer> right) {
    int i = 0, j = 0, k = 0;
    while (i < left.size() && j < right.size()) {
        if(left.get(i) < right.get(j)) {
            list.set(k++, left.get(i++));
        } else {
            list.set(k++, right.get(j++));
        }
    }
    while (i < left.size()) {
        list.set(k++, left.get(i++));
    }
    while (j < right.size()) {
        list.set(k++, right.get(j++));
    }
}