I skipped a few grades, so the school system set me up with a math mentor to make sure I wasn't skipping any basics. First day, the so-called mentor told me I obviously wasn't gifted in math. I was seven years old, and what I heard was, "You're horrible at math." Completely changed the course of my life, causing me to avoid as much math or math-based science as I possibly could.
Fast forward a couple of decades, to when I was working as a curriculum developer. I had to develop lessons based on Euclidean geometry. Aack!!! But I dug into it, and I discovered it actually made a ton of sense to me. That's when I started questioning what the math mentor had said all those years before. Ended up teaching myself calculus over a weekend and went on to earn a graduate certificate in astrophysics with straight As. I probably won't actually switch to a STEM field at this point in my life, but wow, how could my life have been different? Words matter.