I’ll go first: “You have to have children when you’re young,” told to me when I was in my late 20s, with no desire to ever have kids, and no means to support them, by someone divorced multiple times with at least one adult child who does not speak to them.
Also: Responding to “How do I deal with this problem?” questions with “Oh, don’t worry about it, it’s enough that you’re even thinking about it!”
I was in a “troubled youth” cult for many years. “Unhelpful advise” is an understatement compared to the heaps of fear-driven doctrines and rituals we had to follow, lest be homeless. I could go on for days about this topic, but the biggest “unhelpful advice” was to cut all of my childhood friends from my life completely, on the basis of “my own good.” I am now a virtual stranger to them, and although I’ve somewhat made amends, nothing will ever make up for our lost years.