In Eastern Orthodoxy, when doing a service, do little mistakes really matter, and should they be cause for concern? I have a question about the Ecclesiastical Typikon regarding the singer of the church. Does it really matter (i.e., it is a big problem) if he fails sometimes or instead of a song he sings another one? I am referring to a small mistake, like: at the Theotokion the singer must sing, for example, the Theotokion for Transfiguration of Jesus (on the last day of the afterfeast), but instead he mistakenly sings It is Truly Meet. Is there any problem?

Is there any Saint who has spoken about this kind of involuntary mistake? And also about if the Typikon (and all the Holy Services books) should be complied very very precisely (or maybe is more useful to pray much better)?
You clarified your question in a comment:


  If there are a dogma regarding that, if it is considered a sin, if God
  and/or The Church focuses on very precisely compliance of Typikon or
  Orthodox Tradition, if it does not matter a such kind of mistake in
  generally


Sin, as John wrote, is lawlessness (1 John 3:4).  This is also the definition cited in The Longer Catechism of the Orthodox Church, by Metropolitan Philaret.  The 20th century Orthodox theologian and monk, Justin Popovic explained this verse:


  The law of God is the Gospel:  lawlessness is sin.  Its [sin's] only
  law is to be without the law of God and against the law of God.  In
  essence, the law and that which is lawful is only that which is divine
  and of the Gospel; lawlessness is everything that is in opposition to
  that which is divine and of the Gospel.
  
  Commentary on the Epistles of St. John the Theologian, (tr. Sebastian Press, 2009), p. 40


In the Gospel, we are commanded to be perfect (Matthew 5:48).  In this sense anything we do or fail to do in dishonoring or honoring God and our brothers and sisters through inattention is sinful.  The prayers and rubrics in the Typicon were prescribed by our hierarchs for the spiritual benefit of all who read and attend the service, so we fall short of helping our brothers and sisters when we make mistakes in the service through our inattention.  But we also fall short when we have an unkind word for them or an unkind thought about them.  There is not a special checklist that God maintains that relates specifically to errors of one sort or another.