"False Prophet" is a term that gets bandied about quite often in some circles. But what is a false prophet? To some, it's pretty much anyone who doesn't accept their eschatology! When the Bible uses this expression, it has a specific meaning.
A false prophet is NOT someone who misses a prediction. I could look at data & analyze trends, review history, consult experts, construct a hypothesis and make a prediction based on the available data & my understanding of it. If I say, "I think the New England Patriots win will the Super Bowl this year" and they don't even make the playoffs, am I therefore a false prophet? No, because I was just telling you WHAT I THINK, and made it clear that's what I was doing.
A few years ago, comedienne Tina Fey did a skit where she impersonated Sarah Palin and said, "I can see Russia from my house". People got confused and thought that was an actual quote from Palin. She never said anything like that! And Palin never authorized Fey to speak on her behalf. What Fey did to Palin is just what the false prophet does to God!
A false prophet claims to speak for God, saying, "God told me" or "thus saith the LORD" as if God spoke in their ears a word to give the rest of us. They act presumptuously! Just as Sarah Palin never gave Tina Fey permission to speak in her place, so God never called or anointed the false prophet to speak for Him. "I sent them not, yet they ran" Scripture says (Jeremiah 23:21).
Do you even know what a true prophet looks like? He or she will be heart-broken over the sin of God's people. When they intercede, they include themselves in confessing the sins of their people. The true prophet fears God, and is careful what he says & how he says it. The true prophet will warn people to turn from their sin.
A false prophet, on the other hand, denies Divine judgement on sin is coming! "It's OK; God understands; we're only human, it's natural, blahblahblah!" A false prophet condones sin, excuses it, allows it. He or she uses the same logic as the serpent in the Garden: no, God didn't really mean that, nothing's going to happen, go ahead, have fun.
Under Mosaic Law, a false accuser would receive the penalty for the crime he accused the innocent person of. So be careful who you call a false prophet: it's a serious charge, and you'd better be able to back it up.