"Mad Men" creator on Le Cirque mistake: "We made a terrible error"

Joan Harris (Christina Hendricks) in a scene from "Mad Men."

On Sunday's episode of "Mad Men," Joan (Christina Hendricks) mentions that she made reservations at Le Cirque.

One problem -- the famed New York City restaurant wasn't open yet.

It's a rare historical error for the '60s drama, one that creator Matthew Weiner copped to on Tuesday. The sixth season takes place in 1968, and Le Cirque was opened by Sirio Maccioni in 1974.

"We make mistakes," Weiner said during a discussion at The Paley Center for Media in New York. "And in this case -- Christina's not here, but she was ad-libbing," he joked.

"No, it was a terrible error," he admitted, adding, "and I think that the pleasure some people derive from this mistake makes it more entertainment for the show."

Not that Le Cirque minded the gaffe one bit - a publicist for the restaurant told The Hollywood Reporter: "Sirio would have been open if he knew that Joan wanted a reservation in 1968."