AddListItem(list_item_t **pfirst, LPVOID data)
{
    list_item_t *new_item = malloc(sizeof(list_item_t));
    if (new_item == NULL)
    {
        return ERROR_OUTOFMEMORY;
    }

    new_item->next = *pfirst;
    new_item->data = data;

    *pfirst = new_item;
    return NO_ERROR;
}

The product calls free() twice on the same memory address, potentially leading to modification of unexpected memory locations.