When waking a computer from sleep or the screensaver, get in the habit of using an arrow key, shift key, or moving the mouse. Other tempting wake-up signals, such as hitting any key or clicking the mouse, risk triggering an unwanted, potentially dangerous operation if the computer is not quite asleep. The display might be blank because the monitor is unplugged, because the monitor has a built-in energy saver, or because a laptop booted up thinking that an external monitor was connected. In any of those cases, you might have a live computer listening to a live keyboard or mouse; when you intend to say "wake up, computer," you might actually be one-click purchasing a 30" Apple Cinema Display on amazon.com. (If you tend to browse amazon for such things, that is.)
It's farfetched, I know, but this is another "if it happens just once, you're screwed" events, easily avoided by routinely waking up a sleepy computer by (my personal favorite) hitting up-arrow.