Gothamist: Some Grand Central Terminal Secrets Revealed:
Some things are hidden in plain sight in the Terminal. One is probably one of the biggest mistakes ever – the impressive mural of the October zodiac in the Main Concourse was painted on the ceiling backwards. First noticed shortly after the Terminal opened in 1913 by an astute commuter from New Rochelle, the New York Central just claimed it was the view from the heavens, so they didn’t have to redo the ceiling. The great mistake was never corrected, even after the MTA cleaned decades of tar from cigarettes of the ceiling and restored it. The MTA also didn’t fix the small hole in the ceiling caused by the display of a Redstone rocket in 1957 which someone forgot to see if it would fit in the building.
There's also a secret communications system in the Terminal in front of the Oyster Bar on the lower level ramp. The Guastavino tile archway is actually a whispering gallery: Just stand in one corner and talk, while the person in the opposite corner can hear you perfectly.