Adventure · Atari 2600 · 1980
A secret room containing the message "Created by Warren Robinett" — the first Easter egg in video game history.
Atari had a policy of not crediting game developers on cartridges or in documentation, fearing that competitors would poach talent. Warren Robinett, the sole programmer of Adventure, hid his name in a secret room accessible only through a specific series of steps involving a tiny one-pixel dot. Robinett had already left Atari by the time the egg was discovered, making it impossible for the company to remove it before the cartridge shipped in massive quantities. Rather than remove it, Atari embraced the idea, and the discovery inspired virtually every game developer who followed to hide their own secrets in software.
Find the invisible one-pixel 'grey dot' in the Black Castle's catacombs and carry it to the corridor just east of the Yellow Castle. With the dot and at least two other objects in that room, the eastern wall becomes passable, leading to a room that displays "Created by Warren Robinett."