Foreign adversaries buying ninja costumes in 3...2...1...

from here and here

It seems to me that if you're part of an elite military force you probably should be skilled enough to deal with a lone kook with sword and a costume. Otherwise your enemies abroad are going to get ideas.         

When you don't want people to know how many helpings you've had

from here and here (image source)

There should be no shame in going back for 2nds or 3rds or 4ths or ... Well, maybe you're exceeding the limits of other peoples' hospitality, but no matter - how much you eat is nobody's business but your own. Protect your dietary privacy and sneak some extra helpings.

Don't bring a shuriken to a car-fight

from here and here (image source)

 If someone is coming at you with their car, a tiny bit of metal thrown from your hand isn't going to do much. In a battle of car vs. ninja throwing stars, the car wins.