Zac Posen threw a diva fit at the Bergdorf Goodman anniversary party at the Plaza after being asked to be a part of a group of young designers singing a salute to the store on stage.

As the Fifth Avenue retailer celebrated its 111th anniversary at the landmark hotel Thursday night, a group of designers gathered on the stage to sing “Happy Birthday.” But sources told us Posen “threw a fit” on learning he’d be joined by Jason Wu, Prabal Gurung, Thom Browne and Adam Lippes.

“Bergdorf included all of the designers to sing ‘Happy Birthday,’ ” one source said. “But Zac thought it would be just him, Carolina Herrera and Oscar de la Renta. He was upset Bergdorf included other designers. He was a total prima donna.” We’re told Zac stormed off the stage and left the party.

But another witness said Posen just got tired of waiting for the singalong to get organized. “Zac was near the stage, waiting for the other designers to gather,” the attendee said. “But after dancing with Coco Rocha for 20 minutes, he grew tired and decided to leave.”

Meanwhile, Hilary Rhoda, Karolina Kurkova,
Roberto Cavalli, Olivier Theyskens,
Anna Wintour, Glenda Bailey, Joe Zee, Tara Subkoff and Martha Stewart all packed the event, while Bergdorf Goodman’s Linda Fargo presided over the party in an Akris gown bearing an image of the store.

“Watch the trains up there,” we were warned by staffers on arrival, but guests had varied success.

BLK DNM designer Johan Lindeberg went flying toward a couch after tripping over one girl’s long blue train. Cynthia Rowley carefully jumped over the hurdle. Singer Rita Ora’s white Pucci halter gown was stepped on by a woman near the bar. Pucci artistic director Peter Dundas stood next to her and straddled her train to protect it,

Down near the dance floor, Jay Manue
l held a woman’s dress hostage after he stood on it for 15 seconds. “I was being so careful about the trains,” he said apologetically.