My helmets are all pretty much stock except for the small Triumph raised small Union Jacks. But my baby brother has one of those german helmets ( I hate them) from his youth that has alot of stickers.
Some tastless examples:

Does this codom make me look fat?

If you are what you eat- You could be me in the mourning

I'm not arrogant I'm just better than you

Friends come and go but enemies accumulate

Better a sister in a whorehouse than a brother on a rice burner

Luckily he's grown up, in his forties with wife and a daughter and rides a new Tiger. His helmets are tame like mine now and his daughter is probably why his old german helmet is hanging in my garage not his.
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Every normal man must be tempted, at times, to spit on his hands, hoist the black flag, and begin slitting throats. H. L. Mencken