Some examples of prophecy in the Bible to show that prophecy is completely from God, miraculous, infallible and authoritative, BUT not adding to Scripture:
1 Samuel 10.25: Samuel actually wrote the duties of the Israelite king in a book. It was probably an exposition and application of Deuteronomy 17. It certainly is not recorded anywhere in Scripture.
2 Samuel 12.1-12: Nathan prophecies to David, telling him a parable about a wicked rich man, so that David will be convicted of his sin with Bathsheba. The prophecy is obviously recorded in Scripture, but when Nathan said it (the actual prophetic event) he was not speaking Scripture.
Acts 21.10-11: the same would apply to Agabus’ prophecy. He was a prophet; he made an authoritative claim that came true. But he was not speaking Scripture. The prophetic event is recorded in Scripture.
1 Corinthians 12-14: the church in Corinth also had prophets who prophesied, though their prophecies are not found in our Scriptures.
What is the point here? It seems a bit naive to argue that prophecy cannot exist simply because the canon is closed.
At the same time, it cuts against the very nature of prophecy to say that present day prophecy is fallible, as Wayne Grudem might say. A thorough study of prophecy in the Bible shows that prophecy, much like we might conceive of true preaching, is the authoritative proclamation of God’s Word meant to convict, edify, comfort and exhort God’s people.
To Cessationists, please do not let bad Charismatic practices influence your interpretation of Scripture.
To Continuationists, please do not let a prosperity theology creep into your understanding of how God chooses to give certain gifts (i.e. more faith, more pursuit, more earnestness, etc. equals more miracles).