Unlike the premeditation of evil it is when dark thoughts UNWILLINGLY get into your mind causing problems. For instance this can be if you have a long task list, or things like the job economy stress you out even though they only exist in the future - probably because you cannot solve it right now!
The problem is that worrying about something often consumes so much energy that it makes the performance worse...
It is a signal your brain wants to get rid of the problem quickly