When your screen saver kicks in because you haven't done anything on the computer for too long, the following behaviors are non-optimal:
1. Hit the space bar -- since you probably don't know where the focus is, you don't know what this is going to do.
2. Hit the Enter/Return key -- this is even more dangerous than the space bar.
3. Tap the mouse -- this is safe and it works most of the time, but it sometimes doesn't work because you don't move the mouse enough, so you might have to do it two or three times. Not efficient.
4. Put your hand on the mouse and shake it -- this is safe and it works all the time, but it's highly wasteful of time and energy.
Just hit the shift key (or Alt key or Ctrl key). Those are always safe (well, unless you are using an application written by an idiot) and they are highly time- and energy-efficient actions.
This useful computer tip has been brought to you by someone who gets irrationally annoyed at other people's non-optimal behavior.