I'm smiling, shaking your hand. "Yes, this was a very productive conversation! I'm sure glad I met up with you and we had this discussion!"

You think I'm going to help support you in your efforts to do whatever you're going to do with the ideas or information you've shared with me tonight. Instead, I'm going to report it to another group that will stop you from doing what you want, take your ideas, or exploit the information in a way counter to your goals.

I've just lied by breaking the maxim of Manner.

[Edit: It's arguable that breaking Quantity would also cover this--I didn't say WHY it was productive--but it seems clear to me that it's essentially a violation of Manner.]