My example: I remember in one of my early education classes (almost 20 years ago) the professor would always compliment students on their practice lessons. Every student received a compliment about the content of their lessons. Except me. Mine was always about how great my voice was. Never a comment about my teaching/speaking ability like everyone else. This was so insulting to me, I have never forgotten it. The professor probably thought he was encouraging me. Wrong.
Student example: A few years ago I was sitting in a restaurant when I noticed a young woman staring at me. I figured she was just trying to flirt with me or something like that. No. She came over and asked if my name was Campbell. When I said yes, she said that she had been in my class over ten years ago. I supposedly told her back then that she had too much of an attitude problem to become a teacher. Well guess what, she had successfully earned her teaching certificate and she had been teaching for a while. She left me with a not so complimentary "So f___ you." Some people just don't appreciate motivation and tough love.
Be careful what you tell your students. They might just remember it.