I've often seen the phrase lex orandi, lex credendi translated as "the rule of prayer is the rule of belief." Mike's translation is a little different: "the law of prayer is the law of belief." His translation implies something entirely different than the former. While both indicate a norm, the former has a hint of flexibility to it. We even have aphorisms in English associated with rules: they're made to be broken, and there are exceptions to every one of them. Not so with a law. Laws are not made to be broken, and doing so has consequences.
When Deacon Harold Burke-Sivers was here in December for a conference at our parish, he pointed out that altering the words of the liturgy is altering God's word, as the words of liturgy are taken from scripture. That point hit home with me back then. However, Mike's formulation goes even further: to alter the words of the liturgy is to break the law—not just a rule or a norm. He also makes an important point concerning heresy and liturgy, reiterating what Irenaeus posited in Against Heresies:
As the law worked to build up the Catholic Church, its violation brought about the destruction of the heretics. Irenaeus noted that sectarians' missteps in doctrine often emerged as irregularities in liturgy as well. The Ebionites rejected the two natures of Jesus, divine and human, and held that Jeus was only human. So they refused to offer a mixed cup of wine and water—for the Church saw the mixed cup as a symbol of the god-man. In clear contradicton to Jesus' model and command, the Ebionites offered only water in their eucharistic chalice (Against Heresies 5.1.3). Marcion, for his part, held on to the Church's liturgy; but, in doing so, he committed himself to doctrinal inconsistency, for the Lord took bread and wine—things of creation, which Marcionite doctrine rejected as evil—and affirmed them to be his own sacred body and blood (Against Heresies 4.33.2).
The lesson of Irenaeus is clear: Those who tampered with the Church's doctrine inevitably butchered the liturgy as well. And, in doing so, they cut themselves off from the ordinary means of salvation, the sacraments instituted by Christ himself.
Of course, I can think of a number of areas where spontaneous changes in the liturgy overemphasize one aspect of Truth over another—and that is, in essence, the nature of heresy. The truths of our faith often involve parodox, and in our struggle to reconcile these, we can overemphasize one side of a paradox over the other, or even exclude the "opposing" side altogether. Here are a few:
The Church is the people/The Church is the Hierarchy
Men and women are equal/women can't be ordained
Christ was human/Christ is|was|always will be God
Mary the Blessed Mother/Mary, ever virgin
When we take only one side of any of these doctrines, we distort the teaching of the Church, and that has consequences that affect our prayer lives (as well as our proper spiritual and moral formation).
One of my biggest concerns when I go to Mass is that I will have to correct something that my daughter (or my friends as visitors) will hear the priest say. On one occasion that I invited one of my dear friends (you reading, Theri?), the priest commented that saying "Amen" when receiving the eucharist was to say "I am the body of Christ." (Yes, I was mentally banging my head on the pew with that one.)
So when priests spontaneously change the liturgy or change it after personal reflection, they're putting their own interpretation of the law or prayer over and above the law of the Church. That is an exercise of magisterial authority, but it isn't legitimate authority. We should not only be suspicious of what such alterations imply; we should question those who make them.