For no prophecy was ever produced by the will of man, but men spoke from God as they were carried along by the Holy Spirit.This would seem to be a fairly straight forward NT definition of how prophecy works that would rule out a fallible prophetic gift. With this in mind let's examine the case against Agabus.
We meet Agabus in Caesarea where he intercepts Paul and his team who are on their way to Jerusalem having concluded the 3rd Missionary Journey (3MJ). As Paul has made his way through the cities of Macedonia and Achaia, numerous prophets have warned him that he will be face prison and hardship in Jerusalem (Acts 20:23). As Paul gets closer to Jerusalem he is warned again in Tyre (21:4) and in Caesarea by Philip's daughters (21:9) before Agabus reaches him. Apparently Agabus had a prophetic vision while in Judea and felt compelled to travel north to warn Paul as well. Agabus' prophecy provides the most detail about what awaits Paul and includes the OT practice of having the prophet act out some part of the prophecy.
While we were staying [in Caesarea] for many days, a prophet named Agabus came down from Judea. And coming to us, he took Paul's belt and bound his own feet and hands and said, “Thus says the Holy Spirit, ‘This is how the Jews at Jerusalem will bind the man who owns this belt and deliver him into the hands of the Gentiles.’” When we heard this, we and the people there urged him not to go up to Jerusalem. Then Paul answered, “What are you doing, weeping and breaking my heart? For I am ready not only to be imprisoned but even to die in Jerusalem for the name of the Lord Jesus.” And since he would not be persuaded, we ceased and said, “Let the will of the Lord be done.”(Acts 21:10-14)
In this account as in the others those who are with Paul, including his 3MJ teammates, are urging him not to go to Jerusalem. The Holy Spirit has certainly made it a priority to warn Paul that he will endure hardship if he goes. As we study this some questions arise:
- Did the Holy Spirit want to discourage Paul from going to Jerusalem?
- If not, why the repeated warnings about Paul being imprisoned?
- Given the warnings, why was Paul so insistent on going to Jerusalem?
When Paul did go he was arrested at the temple. Luke provides the details of that account in Acts 21:27-36. Based on that account Grudem assesses Agabus (p78):
the events of the narrative itself do not coincide with the kind of accuracy that the Old Testament requires for those who speak God's words. In fact, by Old Testament standards, Agabus would have been condemned as a false prophetThis is based on two observations. First, the Jews do not bind Paul the Romans do (Acts 21:33,22:29). Second, the Jews do not hand Paul over to the Romans. The Jews wanted to kill Paul and he was resuced by the Romans. Grudem reconciles the "Thus says the Holy Spirit" that precedes Agabus' warning by claiming that Agabus added his own information to the actual prophetic message from the Spirit.
However the account does not end there. Paul through his various defenses recounts what happened when he was arrested. Claudius writes a letter to Felix recounting the events (Acts 23:26-30). While he describes the events similar to Luke, he does note that the accusations of the Jews are what keep Paul imprisoned. Later Festus, bringing Paul before Agrippa will note that there are no charges that he can write when he sends Paul to Rome based on his appeal (Acts 25:24-27). After hearing Paul's defense Agrippa agrees that Paul should be set free (Acts 26:30-32).
From these accounts, Agabus can be cleared of one mistake. The Jews did deliver Paul to the Gentiles. It was their baseless charges and attempts to kill him that kept Paul in custody for 2 years before being sent to Rome. When Paul reaches Rome and recounts the events he says that he was delivered as a prisoner into the hands of the Romans because the Jews objected to his release (Acts 28:17-19). Is Paul wrong in describing the events this way? If not, then neither is Agabus.
On the second mistake, it must be acknowledged that it is an argument from silence. None of the accounts describe Paul as being bound by the Jews. However the accounts do not say that he was not bound by the Jews either. Given that Paul was seized and dragged outside the city to be beaten and killed, it is possible that the Jews bound him. Since it was a mob scene they may have even used something quick like a rope or Paul's belt. That would clear Agabus of his second mistake. 
Is this view without difficulties? No. However, I think it best deals with the information provided in Acts and what we know about OT prophets. This view allows the phrase "Thus says the Holy Spirit" to be used in its more normal sense of introducing a prophecy that carries with it the full weight of revelation from God. It also uses Peter's definition of a prophet to align with the narrative - Agabus moved by the Holy Spirit spoke without adding fallible human details.
 Edgar defends Agabus in "Satisfied by the Promise of the Spirit" p 81-84. His presentation spurred me on to look at this further and this post is indebted to his work