You wouldn't believe the truth
Someone tells another person the truth, but that person doesn't believe them
A character is hiding a secret from everybody else. When someone questions them about what secret they're hiding, that character reveals nothing but the full and honest truth, which sounds so outlandish and unexpected that they aren't believed anyway, and the questioning character drops it. For example, Clark Kent reveals his secret to Lois Lane: http://www.youtube.com/watch?v=d0QVvbhMm24 And the trope namer, Jack Sparrow: http://www.youtube.com/watch?v=TNs5j3mSaiw
Hello, Unknown Troper. You'll need to get known to lend a hand here.