I don't think anyone actually noticed this, but:
The MC is supposed to be Japanese. However, in Page 4, he makes the sign for "3" using the European style instead of the traditional Japanese finger-counting method. Then on Page 6, he makes the sign for "2", but this time using the American system.
Of course, this can simply be explained away by saying, "That's the way finger-counting is done in the Demon King's world," but I think it's something worth pointing out as a reminder that finger-counting styles differ from country to country.
I use the American system, by the way.