I got this this idea from a post over on Lifehacker.com. What have you suddenly discovered you were doing wrong?

I discovered I was lifting weights wrong when a guy with huge arms came up to me in the gym and said "hey man, you're lifting way too much weight. Look at me, I only do 25 lbs, and I'm big." I was attempting tricep push-downs with something ridiculous I could hardly do. I meekly mumbled "thank-you" and now I work out with appropriate weights.