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.
• Play a game, setting a new high score that is either 37,000, 73,000 or 77,000. The digits for hundred thousands, hundreds, tens, and ones may be anything.
• Kill off all of your remaining lives, but your last death must be by falling.
• Then set the game difficulty to 4 (press the option button 3 times.)
• Wait for the game to cycle through the demo screen where Kong jumps across the screen, then at the title screen, the programmer's initials, LMD, will appear. (Pictured above)
That is a set of conditions so specific I can't imagine anyone discovering the egg without prying apart the code, much less knowing how to repeat it. Hodges shows how he found it in the code, using an emulator.
I think the only question remaining is why Dyer made the egg so hard to discover. Certainly other Atari-programmed games of the time had Easter eggs that were not only easier to find, their methods were widely known and circulated.
Donkey Kong Lays an Easter Egg [donhodges.com via Game Set Watch]