YES, you can do damage. I accidentally unplugged my ps2 mouse from my Abit mobo and after plugging it in discovered that it no longer worked. I ended up having to use a serial mouse that I had laying around. Now whenever I work on my comp or someone else's I shut the comp down, pull the power cord, then pull the mouse, keyboard, or what ever needs removing. Today's mobo still have current running through them even though it's shut down, you have to pull the plug to totally shut them down. (sorta like a tv, you turn it off with the remote but there's still current active in it)