Blasphemy is speaking evil of/about someone. Like saying "God is a...[bad name]".
If someone speaks something that is untrue, they are probably guilty of lying or at least being wrong, but not necessarily of blasphemy.
If someone speaks something in the name of the Lord that God did not say, that person might be guilty of false prophecy but again, not necessarily of blasphemy. (Unless during their statements they something like "God is bad".)
Blasphemy is a specific type of innappropriate speech.
Love in Jesus,
Kelby
It is giving occasion for the enemies of God to blaspheme. Unbelievers see and hear these claims and it gives them occasion to ridicule Christianity as a whole, when they do not see these things coming to pass.
And it is definitely taking God's name in vain. Vain means no good use. Saying things in God's name, with his authority, that are false is misusing his name.
If I said that God gave me a message for someone, and God did not in fact give me a message for someone, this is taking God's name in vain.
This phrase "taking God's name in vain" is not limited to using curse words. In fact, I would rather see someone use a curse word than claim they are speaking from God when, in fact, they are uttering the imaginations of their own evil hearts. At least people know that those cursing are not claiming something came from God when it did not.
If prophecies are false, and do not come to pass, then false prophets have attributed something false to God. Something God never said at all.
And, people who make a habit of this should be cast outside of the church, after appropriate levels of church discipline.
In reality, though, they will just go to another naive church who will listen to their nonsense. That is the reality in today's world. Church discipline doesn't really exist overall within the Church.
By the way, I think the major reason why they do it is because they want to be thought of as some great power of God, like Simon Magus of Acts 8. To me, though, they impress me not one bit. If someone utters a prophecy, claiming God gave them this prophecy, I am more likely to label them as a non-believer.