This is a subject that has come up often on Yahoo Answers.
http://answers.yahoo.com/question/index?qid=20140524052943AAYDs9v
http://answers.yahoo.com/question/index?qid=20140709120223AATSwt0
http://answers.yahoo.com/question/index?qid=20140831000159AACV2LO
JWs typically respond to the charge by using ridiculous technicalities to fudge the definition of what it means to be a false prophet. Their responses are typically as follows:
~ They would say that they never came up with any original predictions but were only explaining existing prophecies in the bible. In other words for them to be accused of being false prophets they have to come up with a completely original prediction that's not based on anything in the bible. Of course this argument is ridiculous. The bible does not give such technicalities as a basis for excusing some from being guilty of false prophesy. Also at Luke 21:8 Jesus indicated that persons would come in his name and make false predictions about the timing of the end. Such persons are obviously not making original predictions because the bible speaks about the end. And given that they come in Jesus name it stands to reason that they profess to be christian and would likely base their bogus predictions on existing bible prophecies by twisting scriptures. Thus Luke 21:8 implicitly invalidates this particular technicality.
~ They would say that they never made their predictions in the name of Jehovah, that they never said 'these are the words of Jehovah...' or something to that effect. But does a prophet have to explicitly spell this out? If a prophet deliberately gives his audience the impression that his message is coming from God, while not explicitly saying so, is he any less guilty of speaking in God's name? The Watchtower has repeatedly and indisputably given the impression to readers that what they print is not coming from men but from God. There are statements in the literature about "God's direction through his Word and Organization" which convey the idea that the Organization's teachings and direction is coming from God and not from men. There have been bogus claims that Jehovah is the writer/editor of the Watchtower and no man's opinions are found in the literature. There is the claim that the organization is spirit directed, thus ascribing the organization's actions and teachings as being the work of God through holy spirit. To this day, elders and other representatives of the organization can be heard speaking of the direction coming from the organization as coming from Jehovah. At the recent annual meeting one Governing Member, while speaking about all the changes taking place in the organization, was heard saying that even the Governing Body has trouble keeping up with Jehovah's Chariot. What was he implying? He was implying that the changes are not coming from men - not from the Governing Body - but from Jehovah himself. All of the foregoing examples show that Watchtower implicitly claims to act in Jehovah's name. They implicitly ascribe all their doings as the doings of Jehovah. It is therefore disingenous for JWs to claim that their false predictions were'nt made in Jehovah's name. Hypocritically, Watchtower understands that baptizing someone in Jesus' name does not involve having to explicitly say 'I baptize you in Jesus' name' when immersing the person. So how is it they don't apply similar logic to making predictions in Jehovah's name? Shouldn't they see that it's also possible to implicitly make false predictions in God's name without explicitly spelling it out?
One of the most damning indicators of Watchtower's guilt as a false prophet is their own Reasoning Book. Just read through their section on false prophets and see how they conveniently invent their own rules of what it means to be a false prophet - rules that conveniently omit focusing on the aspect of making false predictions. It is very telling that throughout that entire section, not once is the very pertinent Deuteronomy 18:21,22 discussed! Why did they avoid those verses? Because they know they fit the bible's description of false prophets and so they didn't dare discuss those verses. In desperation they had to concoct their own definition and rules so as to hide their own guilt.