I just thought of one.
I can't stand it when someone does something like shaving half of their hair, and dyeing the rest a variety of colors, then rationalizes it by saying "Do it once in your life", meaning it's okay to do stupid or crazy things if it's only once, no matter what it is because you live once.
Before you say that you never seen that happen, the freshmen class at my university dyes their hair (it was usually bleaching or dyeing one color in my undergrad days, but more radical these days) and wears it for several weeks until "bald day", when they shave their hair. Every spring, the university is full of 18-19 year old bald female co-eds. Neither I or my wife had done that, though the wife dyed her hair blond.