As a student of french, I sometimes answer with a "merci" and not a "thank you." Don't do it on purpose. It just comes out. So I do it to this man working the conference who I had spoken to the day before.
"You speak french?" he says in shock.
"Well, a little," I say. "I am learning it but can do okay in a conversation."
"Why didn't you tell me this *yesterday*?" he demands.
"Ahhh ... " I am thinking. "Didn't really come up."
He explains. "I had tickets to the french ambassadors house last night for a formal dinner. My date got sick the hour before I left and I had to cancel. If you had told me you spoke french, I had your card form your book order!! I would have called you!!"
Yes, I would have said no -- the whole happily married for 26 years part -- but oh, to get so close to dinner with the french ambassador at this house!!