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!”
“Just be happy” to a depressed person
Oh wow, jeez, thanks, why didn’t I think of that earlier!
Ah yes, the good ol’ “Just get over it” technique that is supposed to work for any mental health condition.