Mine was a lie that became a truth. "Don't do that. Everybody will laugh at you."
When I was a kid that one line, repeated ad nauseum by my mother and older sister, turned me into a nervous, self-conscious introvert. As an adult I don't care who laughs at me, and am always happy to join in the fun, even at my own expense.
I didn't say it, but someone once told me in my mid 20's after a break-up with my life love, "God won't give us any more than we can handle". I didn't believe it then and sorry but I still don't. I made sure I never said that to anyone.