I wrote a bit about this in an earlier thread about showstopping moments. I attended the first preview of the revival of Nine. No one knew what to expect, but, at the same time, expectations were high because of Tommy Tune's original staging.
When Jane Krakowski made her entrance from the ceiling for "A Call From the Vatican," you could literally hear dozens of people gasping in the audience before breaking into applause. When she exited, upside down and singing her high note, the audience went crazy--SO wild, in fact, you couldn't hear her "ciao" or "I love you Guido." After she was gone, the applause was so wild, it must have stopped the show for two minutes...a particularly interesting feat since the performer who'd garnered the reception wasn't even onstage anymore.
A lesser but still considerably enthusiastic reaction came when Chita Rivera kicked her leg up and landed on Antonio Banderas's shoulder--an image that would later be referenced in The Dancer's Life. I remember thinking, "We've got to stop, or she's not going to be able to get that leg down!"