I'm constantly telling my friend that he's no longer entitled to an opinion on anything, because he fancies Justin Timberlake, which means that his opinion is completly wrong.
Also, I tease another friend about how she's really a man... Mainly it just comes up when someone makes a reference to gender in anyway. Like, "You're a woman, what do you think?" "Oh, she's not a real woman.", but a while ago someone gave out to me for being insensitve to transgendered people, which was something I had never even considered.