I used to think nothing of it until someone said, "it makes you wonder what he was saying the rest of the time".
Now I don't believe for one moment Jesus was a liar, but why did He use that strange expression?
One of 22 examples exists here:
John 16:7 (NIV)
But I tell you the truth: It is for your good that I am going away. Unless I go away, the Counselor will not come to you; but if I go, I will send him to you.