The other being the myriad of influencing factors you can never really isolate.

Say you do the setup from the quote, then one group would also be in a special situation where researchers told them to do this thing, and they would tell their friends, or maybe they would keep it secret, or maybe it would make them a bit weird, or maybe it would pave the way for more addictions, or make them more conscious of it, and they would have another item on their usual shopping list, and they may meet their spouse in a bar, and so on — all of which influences outcome.