1. Alcohol
Probably the worst thing for any diet. Depressant. Slows down metabolism. Beer has lots of carbs. Develop a beer gut. DEHYDRATE your muscles. (Muscles and your body is made up of mostly water). Destroys your liver. Kills brain cells. Makes you sleep with unattractive women. Hangovers ruin your next day and probably make you stay at home and not work out. May cause you to consume bad foods while drunk. Smoke is a close second.
2. Not drinking nearly a gallon of water a day
Misread the title of the thread, so this doesnt apply except to those already on a good diet
Staying hydrated is very important to muscle growth. Also important for maintaining awareness at the gym while working out. (Try not hydrating all day, and then doing some sort of cardio, sweating like crazy and see how dizzy you get) Not consuming water causes bloating, particularly in your face and neck, making you look like you have a fat face as you get older even moreso.