I have to admit, I completely forgot an Iwata Asks for Pokémon HeartGold / SoulSilver was due right about now. This is one of the most interesting ones yet, since they discuss a wide range of topics, including the conception of Mew.
Mew, the enigmatic #151 in the original Pokédex, was shoved into the game well after the debugging process was completed, as a sort of "What if…" measure. Shigeki Morimoto, current director of the Pokémon games, explained:
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!
President and CEO of Creatures Inc., Tsunekazu Ishihara went on to explain that Mew was never intended to be revealed to the public, and was only conceived as something they could use for "post-launch activity," were things ever to reach that stage. However…
But then due to an unforeseen bug, Mew ended up appearing in some players’ games. It looked like we planned all of this, but that wasn’t the case. So although it caused all sorts of problems to many concerned, fortunately enough it ended up having a positive effect.
This resulted in Nintendo announcing their first "legendary offer" in CoroCoro magazine. Mew’s conception turned out to be a stroke of genius as Pokémon’s weekly sales started to match its previous monthly sales after the announcement, and eventually 4x the previous monthly sales per week, due to the buzz surrounding #151. And the rest is beautiful, beautiful history.