Lie Detector

 Bill suspected that his girlfriend, Annie, was being
unfaithful to him, so he bought a lie detector and
asked her, "Tell me, is there any other man besides me?"

Annie replied, "No, besides you, there isn't any other man."

The lie detector confirmed that Annie was telling the truth.

Actually Bill was right, but he had made a mistake - he had
assumed that the person Annie was involved with was a man.