It's definitely happened to me on numerous occasions, and recently. I'm 24 and still struggle with this response, especially since it's always from adults who should know better. I really thought this kind of behavior would have been left behind in high school, but that's not been my experience. I've learned to just look them in the eyes, have this little knowing smile, and say "I'm glad you find that funny, I have a stutter," and they always get super embarrassed and uncomfortable, especially if there's other people around. I understand that stutterers are almost always portrayed as stupid, butt of the joke, type characters in entertainment (think Porky Pig in Looney Tunes), but it still blows my mind when people laugh in my face.