In chapter 28 of the book of Jeremiah, Jeremiah is confronted by a supposed prophet of God, Hananiah. Jeremiah's message to the Israelites is one of judgment by God; however, Hananiah comes along and prophesies a message contrary to what Jeremiah had spoken. The LORD speaks to Jeremiah again and gives him amessage to give to Hananiah. The message confirms what the LORD has already told Jeremiah, thus falsifying Hananiah's claim. That same year, Hananiah dies. I reccommend for the reader to read the account itself for a better understanding of the passage.
What we have here is a clash of proclamations. Both individuals claim to speak for God yet the proclamations conflict with one another. How does one decide who to believe? God did give instruction in Deuteronomy on how to judge whether a prophet is truly speaking for God. The people were to wait and see whether what the prophet said came true. If it did, the prophet spoke from God. If it didn't, he wasn't speaking for God and he was to be put to death.
In our own day, Christians face the same challenge as Jeremiah. Granted, Christians aren't receiving special revelation directly from God as the prophets did, but we have His word written. The Scripture is sufficient for us to know all we need to know about God and His plan of redemption. It's what He has chosen to reveal to humanity at this point in history and we ought not to step outside of its boundaries. However, there are many who do step outside the boundaries.
There are many distortions of God's word which are proffered in our day. I won't go through the list here but there are plenty of individuals and groups which deny key aspects which God has revealed to us in Scripture. One example being those who deny the doctrine of the Trinity. The Scripture is not ours to edit; rather, it is ours to proclaim and defend. I believe it was either Martin Luther or John Calvin who said something like "Just as a dog stands by his master to defend him, so a Christian must stand by and defend God's truth."