So, you want an example a technique that I can argue is harmful, but it is difficult to predict that harm? You want a known unknown unknown? I don't think I can provide that. But if you look at my assessment of the the keeping cookies available trick, I explain how there is some possibility of harm and what kinds of evidence one might use to evaluate if the risk is worth the potential benifet.

No, an example of a technique that is harmful, but whose harm would have been difficult for a reasonable person to predict in advance. The potential downside of the cookie trick is easy to notice and easy to reverse (well, I guess you can't easily reverse gaining epsilon weight, but you can limit it to epsilon), so as a reason not to try it's very weak.

Would it be a good idea to spend some time to figure out if one of the tricks stands out from the other as more likely to work, either generally or for you in particular? Being able to systematically try the trick that is well supported and understood has some value. Or, if in discussing one aspect of the trick that you think would never work, and it turns out you were right, what you understood would not work, and the actually trick is something different, you have not just saved a lot of time, you have prevented yourself from losing the opportunity to try the real trick.

I take my point back. If you can only try one thing, it makes sense to just act if there is only one option, but to demand a good reason before wasting your chance if there are multiple options. (Formally, this is because the opportunity cost of failure is greater in the latter case.) Realistically, "willpower to engage in psychological modification" seems like it would often be a limiting factor producing this effect; still, I would expect irrational choice avoidance to be a factor in many cases of people demanding a reason to favor one option.