What is the most unhelpful advice you have received?
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!"
When I was a teen I worked as a waiter at a dirty smokehouse/bbq place.
One of the kitchen staff there would make sexual comments about me. Say things like "You're lucky you look good because you're so stupid." And would ask what kind of underwear I was wearing.
I told my parents about it, and the advice they gave me was "Deal with it. You need a job."
Within a month that kitchen staff member had started to grab me and sexually assaulted me.