Whats the single biggest mistake you see beginners make?

They fail to understand that they are beginners. Novices, as we call them. They pick a routine out of a magazine based on what the guy doing it looks like, with no regard for the fact that they are not the same piece of physiology as the guy in the magazine. Training is physical activity designed to produce a specific response -- it is stress, recovery from that stress, and adaptation. People respond to a training program based on their own personal training history. The weaker you are, the more room you have to get strong, and the stronger you are through training, the harder it is to get stronger. Novices get strong faster than advanced lifters, so an advanced program is not the best for making the rapid gains possible for a novice. Time is wasted, potential is wasted.