Last year, the coder who wrote the Atari 400 port of Donkey Kong revealed it had an Easter Egg but it's "totally not worth it." Someone has gone to the trouble of finding it.
Donkey Kong for the Atari 400 and 800 was one of the best early ports of an arcade game, and its writer, Landon Dyer, detailed how he built it practically by himself, with no support, reference code, or anything one would expect in a licensing deal. He also revealed the existence of the Easter egg, but incorrectly described how to get it ("something like: Die on the 'sandpile' level with 3 lives and the score over 7,000," he writes.)
Don Hodges, who earlier fixed Pac-Man's notorious kill screen, set to picking apart the Donkey Kong code and finding the egg. He did. It's rather underwhelming, but for posterity's sake, he found the conditions for achieving it.