I have to disagree with that statement. I've seen many posts from people who started with "just a little won't hurt" and wound up off plan for days or even weeks/months. It may not have been popcorn specifically, but pick any food not on a low carb plan that a low carber may want as a "treat" and the result is often the same.
If you can indulge and get right back on track again, good for you! But for someone who was truly addicted to carbs, "just a little" is the kiss of death as Dr. Atkins says and I wouldn't encourage anyone who is not completely sure of their ability to stop at just that little treat to go ahead and have it.
Knowing your limits and your limitations is the key to keeping on plan and just might be the difference between reaching your goal and abandoning your low carb plan and regaining all the weight.
My nickel's worth...