Mine is how people always boast of never using heat on their hair, but get it coloured every 6 weeks 
Looking down on people who use tanning products but seem chuffed about their 'naturally' scorched cell damage.
I stopped blow drying my hair 5 years ago as I fell for the myth that leaving it alone would make it shinier, healthier and longer. It didn't change a thing. I had the same amount of split ends (regardless of trims) and insane frizz. Recently went back to blow drying a few time per week and my hair is smoother and easier to manage now.