Spot the foreigner in the mix!
You're walking to the local train station as part of your daily commute and living in the dream world that is your own thoughts when suddenly, you hear someone (usually a child) shout, "gaijin da!" (a foreigner!) Instead of being wrapped up in the relative peace of your own thoughts, you're pulled into the reality that you are a stranger in a strange land and that people are thinking you don't belong and those with the poorest impulse control are letting you know about it.
I understand that people, and especially kids, will think such things and act on them. I don't hold that against them, but that doesn't mean that I enjoy or am comfortable with being loudly reminded that I'm an alien, and I won't miss it.