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Some shows were cancelled/ended before they broadcast all of their episodes.
These episodes may appear later on dvd or in some other way.
I tried adding an episode to Mind Games http://www.tvmaze.com/shows/4164/mind-games and found that I was unable to set the Airdate to "Unaired", however I could leave it blank.
Perhaps this option should be added. It may also come in handy when adding future episodes, where you know the title but not yet the airdate.