According to Luke, the author of the book, while they were staying in a place for several days, a man named Agabus who could tell messages from God came to visit them. Christians often understand this to mean that God sometimes sends special people to give important messages.
Acts 21:10: And as we tarried [there] many days, there came down from Judaea a certain prophet, named Agabus.
These verses provide a test for true prophecy, suggesting that a prophecy that does not come to pass is not from God, potentially contradicting views on prophecy in Acts 21:10.
Deuteronomy 18:21-22: And if thou say in thine heart, How shall we know the word which the LORD hath not spoken?