1. Halitosis, or bad breath
One of my longtime barbers always ate garlic for health purposes. The only problem with that is he forgot to use unscented, and it became very unpleasant to hold a conversation with him. So make sure your breath is pleasant, or keep your mouth shut!
2. Smoking
It's not a good idea to smoke in the shop or in front of customers that don't smoke, unless you know for a fact that it doesn't bother them.