Basically it may (or may not) cause predictable (or unpredictable) effects.
Imagine throwing a couple of dices, and if you get an odd total, but larger than 7, but only on wednesday nights, and if there is full moon (or even if not) it may (or may not) affect your internet connection.
It is - not so casually - Rule #5:
http://forum.mikrotik.com/t/the-twelve-rules-of-mikrotik-club/182164/1