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 a new dog owner, went to /r/Dogs to ask about a particular behavior my dog was exhibiting I’d never seen or read about before (turned out to be normal tho) and every reply I got basically told me I don’t know how to care for an animal and that I should give him to someone else.
It was then I realized that it wasn’t just /r/RelationshipAdvice that was full of bitter, jealous losers whose advice is always “dump them.” It applied to literally every single subreddit dedicated to advice. They may have started with good intentions and knowledgeable people, but over time filled up with people who had no business giving anyone advice.