The latest "Iwata Asks" bull session regards Punch-Out!!! - the NES original, not the Wii sequel - and it's quite revealing. Especially when designer Makoto Wada (pictured) pipes up.
Wada-san, who came aboard Nintendo right around the time the Punch-Out!!! port for the NES was being created, spills two interesting details. The first is that he created Mario as the game's referee without permission. It was just accepted.
Satoru Iwata: You could get away with a lot then. (laughs)
Shigeru Miyamoto: We didn't have an approval system when using Mario images back then and it went right past my check. (laughs)
Wada: That's why it is a slightly strange looking Mario.
The second tidbit - it sounds like he's describing a protip/easter egg that no one's known, for 22 years.
Wada: This is a great opportunity, so I have something I'd like to say. In Punch-Out!!, the game gives you a lot of hints about effective timing of punches. There is a big boxer called Bald Bull in the NES version as well and a light flashes to the right in the audience when he charges. If you punch when it flashes you will land a body blow.
Tanabe: What? Really?
Wada: No one has known about that for about 22 years…
Wada: I was wondering when I would have a chance to tell people that.
Iwata: You've been holding that information for 22 years since the release. (laughs)
Wada: Now that I had the chance. (laughs) There are a lot of hidden elements in the NES version.
I do know the first time you face Bald Bull in that game, you take him out with a one-punch body blow during his charge. In fact, it's the only way to stop his charging. But I never timed it to any crowd camera flashes. Weird.
The rest of the discussion is a great read on the development of the franchise, and other early 1980s games at Nintendo.
Iwata Asks: Punch-Out!!! [Wii.com]