I believe the Scriptures are infallible as originally written and yet in this passage we have a clear example of a prophetic word that wasn't 100% correct. That is deliberate, because the Holy Spirit is giving us an example of the nature of New Covenant prophecy. We know in part and we prophesy in part (1 Cor 13:9-12). Prophecy was not open to all in the Old Covenant, but is our inheritance in the new. Jesus says my “sheep hear my voice” and Pentecost in Acts 2 opens up all of us being able to hear God’s voice – “all shall prophesy.”
This widening of revelation is a lowering of the bar. Prophetic utterances under the new covenant are not 100% God's words and will need weighing and testing. We need to take hold of the good and throw out the rest. That’s why it is not helpful to prophecy “The Lord says…” because it doesn’t leave room for weighing!
In v4 the disciples tell Paul “in the Spirit” to not go to Jerusalem, but he ignores them. Is he disobeying the word of the Lord? In v11 Agabus prophecies that Jews will bind Paul and hand him over to the Gentiles and yet in v23 we read that it was the Romans who bound him. So Agabus is not 100% accurate in his foretelling.
I think it was John Wimber who described prophecy as made up of 3 components – revelation, interpretation and application. Sometimes we get a revelation and then interpret it wrongly and deliver the prophecy tinged with a duff interpretation which throws off the application.
Perhaps the disciples in v4 rightly got the revelation that there will be trouble for Paul in Jerusalem and then spoke out the interpretation that Paul shouldn't go,
Agabus got the main thrust right – Paul will be bound and handed over to the Gentiles, but got the detail of who would do the binding wrong.
Under the new covenant, We know in part and we prophecy in part. We need to be careful to deliver the revelation and allow the interpretation and application to come as it is weighed. For example you can say “I had this picture… I think it means this…”
Lastly, be careful giving directional prophecies – they are easy to get wrong by adding what we think the interpretation and application should be.