In a debate (or perceived debate) a casual remark that sounds like an insult, and is perceived as one.
Norma: I think we may be getting rain soon.
Betty: Actually, Norma, after analyzing the latest data, including local atmospheric pressure, temperature and cloud characteristics, and the velocity of fronts within a 100 mile radius, I conclude that we will NOT be getting rain soon.
Norma: Well, Betty. It sounds like, when you grow up, you might be quite the mee-tee--a-rol--o--gist.
Betty: Are you calling me FAT?
Betty: You clearly know nothing about weather, and you try to overcome your ignorance by saying that when I grow up, I'll be a MEATY urologist. Ad hominem attacks have no place in rational discussions.
Norma: No, Betty. Your scientific knowledge actually impresses me. I said you were going to be quite the METEOROLOGIST when you grew up.
Betty: Oh, so I thought it was an ad hominem...
Norma: and it was really only an ad homonym!
(meterologist homonym courtesy of Richard Lederer)