The dangers of the double negative...

ATC: Did you get your numbers?

Airline: [Somewhat garbled] Negative.

ATC: Was it "Negative?"

Airline: [Again, rather garbled} Affirmative.

ATC: So ... "negative," or "affirmative?"

Airline: "Affirmative" for "negative..." [The other guy in the cockpit can be heard chuckling in the background.]