Quote from pspr:
gnome, I've seen static ruin many a computer component because someone decided to carry a CPU, hard drive or motherboard across the room without putting it in the anti-static bag. Usually the first few charges just make the component function slower or irratically but eventually one will cause the component to totally fail. And these aren't static charges you see when you rub your feet on the carpet and zap someone with your finger. These are small charges that you don't even notice or feel. This damage occurs quite often outside the shop (and even in the shop)
Small static discharges to the case are harmless if it is grounded. The charge just goes out the house ground wire. But if you did the same by touching the motherboard or other component in the computer there would most like be damage to that component.
Here are some rules to be safe.
1.) Always have your computer plugged in with with power to the case (NOT turned on though) which will provide line ground to the case when opening it.
2.) Always use a wrist ground to case strap when working in the computer. (Or, if not, NEVER, shuffle your feet and don't' work while on carpet, and keep one hand on the case for ground.)
3.) Don't take parts out of the anti-static bag until you are ready to put that part in the computer.
4.) NEVER carry a component somewhere without first putting it back in the anti-static bag.