Note that some spoilers for Ready Player One (the film version) follow, but only as they relate to the subject of the final challenge, finding the Easter egg in Adventure, a 1979 game for Atari 2600.
In Ready Player One, Halliday’s final challenge is not to beat Adventure, but to find the (very well) hidden room with the designer, Warren Robinett’s name emblazoned in it. There’s a fantastic story behind Robinett’s inclusion of what became video gaming’s first Easter egg, but the short version (go read the long one!) is that Robinett wanted to sign his masterpiece, but Atari’s wouldn’t allow it. Atari’s theory was that programmers would be poached if their names were publicized. So Robinett hid his name. Today, you can still find it hidden in Adventure’s code.