Not really a misquote, since the quote itself is generally perfectly accurate (mutatis mutandis). Rather a misunderstanding of what the quote means. In my experience, nine out of ten people who say “Wherefore art thou, Fred?” think that they are using some sort of Shakespeare-speak for “Where are you, Fred?” and completely miss the point that “wherefore?” means “why?”. Juliet is not asking “Where are you Romeo?” but rather “Why are you Romeo?” … and not someone else with whose family my own family is not at daggers drawn.