In France, visiting my penfriend. In a swimming pool, got something in my eyes, and exclaimed, "Ah! Mes œils!"
Cue laughter from my pen-friend and a reminder that the plural of œil is yeux. Oops, I knew that :)
(I also got curious looks when I referred to somebody's grandchildren as their grands-enfants around his family... I suppose it makes sense that your children's children, being even younger than your children, would be petits-enfants rather than grands-enfants, but I was taking my cue from English here.)