When considering the Eucharist itself I appreciate Alexander Schmemann's perspective:
(For the Life of the World, Chapter 2, Part 7)
It is indeed one of the main defects of sacramental theology that instead of following the order of the eucharistic journey with its progressive revelation of meaning, theologians applied to the Eucharist a set of abstract questions in order to squeeze it into their own intellectual framework. In this approach what virtually disappeared from the sphere of theological interest and investigation was liturgy itselfy, and what remained were isolated "moments," "formulas" and "conditions of validity." What disappeared was the Eucharist as one organic, all-embracing and all-transforming act of the whole Church, and what remained were "essential" and "nonessential" parts, "elements," "consecration," etc.
Your question reminded me of exactly this passage, because it you appear to be worried about the legal validity of a service and legal culpability of one who might "mess up". Like Fr. Schmemann says, I think it is first and foremost important to recognize each service as a journey of the Church and not a set of magic words that change the bread and wine into something special (the priest is not Harry Potter and the service books are not spell books). What validates the service is the Holy Spirit and the love present. Note that one of the most curious parts of the litury is "Let us love one another that we may confess, Father, Son and Holy Spirit"; the action of love precedes the dogmatic confession.
That is not to say mistakes in the service aren't detrimental. For instance, kneeling on Sunday or singing the Paschal hymn in the middle of Holy Week is not good, because it violates the how the Church understands those times and uses them to teach us. Also, if a chanter cares nothing for the variation of the services and simply chants the same thing at every service instead of taking the care to learn the cycles then he has failed to love the parishioners by depriving them of the fullness of what the church offers. If on the other hand the parish is trying to learn the services and progress in their understanding in love and under the guidance of their bishop and the teachings of the Church they can do no wrong.