Example from Charles Travis [1] (Pragmatics, pg 97): - Maple trees have red leaves naturally, but Pia paints them green. - One day, a decorator asks her if she has any green leaves for her display, and Pia answers yes. - Another day, a chemist asks her if she has any green leaves for her experiment on green leaf chemistry, and Pia answers no. - In both cases, Pia answered truthfully.
The moral: any true sentence in some context has a context which would make its negation true.