Over hitting the ball or simply just hitting the ball too hard. It's the hard, harder, and hardest approach that gets most of us; you hit a beautiful firm shot, and it comes back, so you want to crank on it a bit more to force an error or hit an outright winner, that too comes back to you with even greater speed/weight/spin, so you really want to send that ball into oblivion by hitting it as hard as you possibly can. 90% of the time, that third shot will go out or into the net cause you're trying to hit it too hard.
Bullfan, "Bull rap, there is video evidence." and "Dude it's on the Internet, and hasn't been hidden."