The story of how Mew (Pokemon) came to be is also pretty interesting. From memory: when they were getting close to shipping, they had a completely full cart (only one or two bytes to spare). They removed all of the debugging stuff, which freed up enough space for a new Pokemon to be defined. A programmer secretly added Mew right before they shipped, and Nintendo didn't find out until a couple of weeks later.
That sounds like a rumor. Mew already existed in the storyline unless that programmer then added the entire Mew storyline (inc. Mewtwo) - otherwise there was no reason for Mewtwo to exist
The character of Mew was meant to be referenced through the story, but not actually catchable. Considering that you can only get Mew by exploiting bugs in the game, it's pretty believable that it wasn't added above the radar.
