Three celebrities from NBC's upcoming "Celebrity Circus" found out the hard way the show isn't all clowning around.

Christopher Knight fractured his forearm while practicing with the German Wheel, a giant disc he failed to control. Stacey Dash cracked three ribs climbing "the silks," a length of fabric suspended 30 feet in the air. And during a rehearsal, Olympic swimmer Janet Evans fell 15 feet from a trapeze.

Evans also caused a wardrobe malfunction for her female partner when her fishnet stockings got caught on her partner's top and pulled it down. "She was completely exposed to the cast and crew," said our spy. "Janet was trying to help her. That's when she lost her balance and fell." The injured were quickly taken care of. The show, which premieres tomorrow, provides the "highest safety measures," said our insider.

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