It won't cause any problems. The main problems I have seen when working with pc's is if you don't unplug the power cord. I was working on a pc with a friend and a static discharge from his finger in the case caused the pc to boot up!!! :-) Was funny as hell cause the discharge made his arm move back.
I even tried putting a whole chess set (magnet pieces) on a hard drive and boot it up and let it run for hours without any problems.
The only thing I had success wiping was a floppy using a fridge magnet (the usually ugly ones you get in the mail ).