Well, you know, bb.com just want to make sure you buy lots of their supps to break through your supposed overtraining.
Most of us know when we're overdoing things in our life. Lack of food, as Markos says, but also lack of good rest and fitness, these will make your recovery take longer.
For me it's a balance thing. I could eat more and train harder, but I don't want to spend my life in the gym, not with me under the bar all the time anyway. I accept slower results as the price of this.
But if you really love being in the gym, or want quicker results, go for it - listen to your body, work as hard as you want.