Dear Reader:
Iwata Asks: Pokemon HeartGold/SoulSilver - Mew was originally added at the end of development
A portion of an Iwata Asks feature with Morimoto, Ishihara and Iwata…
Morimoto: We put Mew in right at the very end. The cartridge was really full and there wasn’t room for much more on there. Then the debug features which weren’t going to be included in the final version of the game were removed, creating a miniscule 300 bytes of free space. So we thought that we could slot Mew in there. What we did would be unthinkable nowadays!
Ishihara: This is in spite of being told after debug ended that you weren’t to tamper with even a single bit! (laughs wryly)
Iwata: What’s the point of going through all the trouble of the debug process if you’re going to go and fiddle with the game afterwards…? I’d venture that this all came from Morimoto-san’s mischievous nature.