Montanism is/was a funny heresy, because it's not entirely clear what it was about and why it was deemed to be heretical. It's especially interesting for modern charismatic Christians, because the problem with Montanus seems to be something to do with his belief in prophecy, which might raise the question: is the modern charismatic movement Montanist?
Montanus started causing problems in the mid-2nd century in the Asian church. According to the sources (unlike many early heresies, we don't have anything written by the Montanists to tells us what they thought, so pretty much everything we know about them comes from people who disapproved of them), Marcion was a recent convert who, wanting to be important, "let the devil in" and started prophesying: "moved by the spirit he fell into a state of possession...and abnormal ecstasy, insomuch that he became frenzied and began to babble and utter strange sounds, that is to say, prophesying contrary to the manner in which the Church had received from generation to generation by tradition from the beginning." He began to attract a following, including two women who also began to prophesy.
The local church got together and decided that Montanism was decidedly non-U, what with its "new fangled" teaching (isn't "fangled" just a brilliant word?) and "madness of the soul." They were kicked out. With them, at some point, went Tertullian, one of the early defenders of Christianity who started off as part of mainstream Christianity and subsequently went off with the Montanists.
Self harm, drugs, and rock'n'roll
So, what was the problem with Montanism? It could be simply that the claim to prophesy was at this point in the church's history considered beyond the pale. However, it looks to me like there are two things in particular that might have been the root of the church's problems with Montanism. First is the manner of the prophesying. An anonymous source talks of the frenzy, babbling and ecstasy of the Montanists' prophecy. This sounds a lot like the sort of prophesying common to the Ancient Near East, but generally not so popular with Christians and Jews. Prophets would whip themselves up into a state of ecstasy, often using music, drugs, or even self-harm to enable them to achieve this, and then go on to prophesy. You find this sort of prophecy in the biblical story of Saul's prophetic frenzy and the prophets of Baal when they have a competition with Elijah to see who can call down fire. In contrast, most biblical prophecies seem to be delivered by people who are rather more compos mentis. So that's one possibility.
The other possibility is that Montanus was claiming to have either new revelation from God or new and authoritative interpretations of traditional teachings. He was prophesying before the formation of the canon of Scripture, and may have been one of the factors prompting the church t come up with an official list of authoritative books in order to rule out new and unorthodox teaching.
Interesting questions, then, for the charismatic/Pentecostal churches: what are the limits of acceptability in terms of prophecy? Where is the border between biblical prophecy and more suspect altered-state prophecy? To what extent is it possible/desirable to create a particular atmosphere/mental state among Christians in order to facilitate hearing from God (particularly, where does music cross the line from helpful to dodgy)? How do we balance prophecy with the teachings and traditions of the Church? Try it: ask yourself or your favourite Pentecostal, "Are you a Montanist?"