In the OT God spoke primarily to a few prophets and their words were weighed. Some believe that's one reason why Jonah didn't want to go to Nineveh because what he was speaking from God wouldn't happen because the people would repent.
Now God speaks to us all (
John 10:27) and wants us to prophecy (I Cor 14:1).
No believer is going to serve God with no errors.. Not one preacher,, teacher, evangelist, prophet, believer is going to serve without error.
False prophets lead people away from God. Matt 24:11, 24; II Pet 2:1
Why "prove all things?" God's prophets do want to hear and speak accurately. What is to be done with a prophetic word, a
thus saith the Lord God?. "'Despise not prophesyings. Prove all things; hold fast that which is good." (I Thess 5:20-21). So what prophets say is to be tested.
Some additional steps are needed to prove all things such as:
-checking it against Scripture (Acts 17:11),
-asking The Holy Spirit and hearing His voice getting confirmation (Matt 18:16;
Acts 9;9-19 especially v 10-12; II Cor 13:1)
-fasting,
-spiritual discernment,
-does it point to Yeshua,
-receiving G_dly counsel (Prov 11:14).
Then when it has been proven, there's warfare in the spirit realm. "This charge I commit unto thee, son Timothy, according to the prophecies which went before concerning thee, that thou by them mightest wage a good warfare," I Tm 1:18