Easter egg (virtual)

A virtual Easter egg is a hidden message or feature in an object such as a movie, book, CD, DVD, computer program, or video game. The term draws a parallel with the custom of the Easter egg hunt observed in many western nations. The earliest known reference to an Easter Egg as a hidden item in a medium appears to be from 1975's The Rocky Horror Picture Show, in which actual Easter eggs are visible in certain shots (under Frank N. Furter's throne, for example), from an Easter Egg hunt the crew had while filming.  Atari's Adventure, released in 1979, contained what is thought to be the first video game Easter egg (the programmer, Warren Robinett's name).
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