Could be making jokes of negation. The same with the sentence: "This sentence is false."
If the sentence is false, the "false" does not apply, apparently. Which means the sentence is actually true. If the sentence is true, the sentence is false, as per its contents.
You can lie about taking a lie-detector test in a similar way.
However, it would be even more interesting when a person lies about taking a lie-detector test, when actually being tested. Does the truth show up then, or does it not?
