Next to flat out injuring yourself at the
gym, overtraining is one of the worst problems with
working out. Overtraining is a result of applying more
stress to your
body than it can
adapt to. As a result your
body releases
cortisol to help adapt.
The cortisol scours your body for protein to break up into glucose. Since the whole purpose of working out at a gym is to build muscle mass (to impress the ladies no doubt), this is completely counter to any muscle-bound caveman's (nothing wrong with being one, however) goals.
Avoiding overtraining is simple, don't live at the gym! Spend no more than about an hour at the gym, and go at most every other day. A proper diet is vital as well.
I'll add some interesting research data on the effects of overtraining as I gather some interesting research data together.