What are the biggest lies ever set to music? I'll throw out a couple of nominees, just to get the ball rolling:
All you need is love. Love is all you need. (This whopper told by Lennon and McCartney. I've never met a landlord or finance company who would take "love" for a monthly payment. Can you depend on "love" for your nutritional needs?)
Love hurts. (I think it was Nazareth who said this. Shouldn't it be that unreciprocated love hurts? Nazareth should have been clearer about what they meant.)
Okay, those are the first two that come to my mind. Yours?