There's no margin for error and the biblical consequences are severe. Just read Deuteronomy 18.
Deuteronomy 18:20–22 (ESV)
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.’ 21 And if you say in your heart, ‘How may we know the word that the Lord has not spoken?’— 22 when a prophet speaks in the name of the Lord, if the word does not come to pass or come true, that is a word that the Lord has not spoken; the prophet has spoken it presumptuously. You need not be afraid of him. (emphasis added)
How do you know if a prophet is really from God?
Well, he's always right.
But what if he prophesies and it doesn't come to pass?
You have two options:
1) Kill the false prophet or 2) Subtly rework the prophecy so that it is now looking ahead to another time in the future. The problem with option 2 in this case is that there was a specific date set which was missed and then another date set which again passed without incident.
Will this be the next Great Disappointment?