As you know, finding mistakes in Mad Men is "one of white people’s favorite activities on earth", so here's my tentative contribution. Don't these drop ceilings look a bit too modern, especially with those air diffusers? Drop ceilings were invented in 1958 and were originally glued or stapled. The T-bar suspension system you see here wasn't invented until the 1960s.
Would love to hear from somebody who knows more.
-- the image is from here