Screwing the computer parts back together is the most dangerous step
I had removed the cover from one of the computers in my office in order to upgrade one of its hard drives (from 20GB to 200GB, woo-hoo). The hard drives are kept in a removable cage, so first I had to unscrew the cage, then unscrew the drive from the cage, then swap in the new drive. Of course, you don't screw everything back in until you've tested...