The world's first easter egg was in the atari 2600 game "Adventure". In it, you pull an invisble object to a certain place, and the programmer's name would flash on the screen. In those times, programmers weren't put in the credits. This guy was obviously pissed off about that.
In the Sonic the Hedgehog cartridges, there is an embossing of the Sonic logo. You can't see it unless you take the cartridge itself apart.