On one hand, to maximize the chances of being able to determine the cause of a problem upon its first occurrence, you want to gather the maximum amount of diagnostic data from the system at all times.

When once you have found a motive the chances are that you will hit upon evidence that till then had escaped you.

If we're not lost in once-upon-a-times, chances are we're projecting ourselves into the future, either worrying about things that haven't happened yet or daydreaming about what might be.