"On a child's Superman costume: Wearing of this garment does not enable you to fly". (i blame the parents)" I've actually seen this one.
"On a bottle of bathtub cleaner: For best results, start with clean bathtub before use. (i guess i'll have to by some other product to clean my tub first before i use the bathtub cleaner...)" My Grandma was seriously pissed when she got her can of scrubbing bubbles on it and it said that very same thing. She wanted to sue the company for false advertising since it claimed that the bubbles would scrub your tub so you wouldn't have to.
I think it is ridiculous that all bags have to say "bag is not a toy. Do not place over head. May cause suffocation." I say let natural selection run its course!