Deuteronomy 18:20
But the prophet who presumes to speak a word in my name that I have not commanded him to speak, or who speaks in the name of other gods, that same prophet shall die.
Jeremiah 5:30-31
An appalling and horrible thing has happened in the land: the prophets prophesy falsely, and the priests rule at their direction; my people love to have it so, but what will you do when the end comes?
And no, the horribleness of speaking a false prophecy did not change in the New Testament, and you were still considered a false prophet if you spoke a false prophecy.