Oh yeah. That happened to me. I remember it like it was yesterday...
I was walking along when I saw a Pikachu. "Ooh! I'm gonna talk to it!" I didn't think I could catch it, since you couldn't really catch overworld Pokemon unless they were legendary or that Drifloon. I expected it to say something like "Pika Pika," but instead it said, "I'm a Pokemon. Let's battle!"
At first I was excited. "Sweet! A talking Pokemon! I'm gonna catch it!" But of course, my dreams were soon crushed as it turned out to be a trainer... Just kidding. I'm not that dumb. But, it'd be funny if someone tried to catch the trainer. XD