I think mine was when I was attempting to say the word "dog" in Mandarin.
Me: Hey, how do you say "dog" in Mandarin? I learnt it last Mandarin lesson but I've completely forgotten.
Ken (exchange student): It's gŏu.
Me: Oh, yeah, I remember now. So it's gou? (I am unsure of tone I used, whatever it was it wasn't right)
Ken: No, no! That's *points to my crotch*.
Ken: Penis! It's gŏu, not gou!
Me: Gŏu! I meant gŏu!