On the subject of mistakes during prophetic ministry, I've wondered what God does about those blessings we pronounce over people that weren't really of Him. And it came to mind that A) Some of these things are still blessings in general, and B) that some of these things could be sinful or create something against God's will for that person's life (if it was brought into being). And as I was thinking about this, I was reminded that Abraham made a mistake and Hagar gave birth to a son. The son should not have come into being, but we see that later God blessed Abraham's mistake because Abraham loved his other son as well. And I think that God may do this in the prophetic ministry as well- that there are some mistakes that God will anoint anyway simply because we meant well, and our heart was for that person. God may also bring our mistakes (wrongly discerned words of blessing) to pass simply for the sake of the person receiving prophetic ministry, so that their belief in the rest of His word that was true does not falter. For example, God may be saying that He was going to increase their impactful-ness in ministry, but since we got a picture of wealth, we happened to mention that God was going to bless them in a new way financially as well. God may not have had that on the agenda, but He may do it anyway simply so the person will believe the rest of His word, and because being provided for in life and being bountiful is what God wants in general for His people.
On the other hand, for B), we see in the Bible that David also had a child by a mistake, but God said that this child had to die. There are some things we say that God simply has to kill, because they aren't of Him, and will be destructive in the end.
But the encouragement is this: That sometimes even our mistakes God may turn into a blessing for the sake of the people involved; and as well, that God may put that extra effort forth to kill those errors that are destructive that we may have accidentally prophesied over someone's life (i'm not saying abortion- aka killing babies- is of God; the reference is merely about prophetic ministry).