Table manner wise, personally, "Inugui" is the likeliest bad manner that we misbehave.
The word "Inugui" means "eating like dogs", which means to eat with your mouth excessively near the table or without holding your dishes or rice bowls. When it comes to Japanese food, it is considered proper way to hold your rice bowls or dishes when conveying the food into your mouths.
Way back, only little children tended to do this. These days, however, even adults often misbehave likewise, because this bad manner makes us feel easy.