Although "Mad Men" won't begin filming Season 4 until next week, fans of the AMC drama got an unexpected bonus when a months-old video of the cast and crew performing a campy version of the 1960 song "Bye Bye Birdie" — a key plot-point — surfaced online. A spontaneous tribute to show creator Matthew Weiner, the video reveals just how many people it takes to recreate the precisely retro show and how few of them can carry a tune. The clip drew mixed reviews: "You may see Rich Sommer ["Harry Crane"] with his shirt off," warns Dave Itzkoff of The New York Times, while  New York's Lindsay Robertson raves: "It's so great we're not even mad at the conspicuous absence of Christina Hendricks and Bryan Batt." Watch for yourself: