There's a moment in "Oz the Great and Powerful," Disney's much-anticipated prequel to the 1939 MGM classic "The Wizard of Oz," where a character falls to the floor, in the midst of a witchy transmogrification into something evil. Off-screen she remains until suddenly, with a heart-stopping smack, a huge green hand pops up and out of the screen (in glorious 3-D), and the clawlike fingernails scrape across a marble tabletop to the agony of any moviegoer's ears.