The funniest and stupidest thing I ever heard came from my dad.
He was talking to my wife, and he meant to say, "I'm not lying." & "This is an honest to God's truth!". He ended up looking completely straight faced at my wife and totally unaware that he just actually said, "Now, what I'm about to tell you is an honest to God's lie!".
I never heard anything so funny in my life!