Doesn't Luke get it wrong when he has Gamaliel say, “Men of Israel, take heed to yourselves what you intend to do regarding these men. 
For some time ago Theudas rose up, claiming to be somebody. A number of men, about four hundred, joined him. He was slain, and all who obeyed him were scattered and came to nothing. 
After this man, Judas of Galilee rose up in the days of the census, and drew away many people after him. He also perished, and all who obeyed him were dispersed.
And now I say to you, keep away from these men and let them alone; for if this plan or this work is of men, it will come to nothing; but if it is of God, you cannot overthrow it—lest you even be found to fight against God” (Acts 5:35-39 NKJV)? This goes against the evidence of Josephus. -- A.C.

Jewish statesman and historian Josephus places Theudas in the governorship of Fadus (Antiquities But Fadus served in this office 44-46 AD -- long after Gamaliel gave his wise counsel to the Sanhedrin. Judas the Galilean, according to Josephus (Jewish War 2.433, Antiquities 18.1-10 and 18.23) rose up about 6 AD, which if an accurate statement puts Judas several decades before Acts 5.

Although you may not find any of the following suggestions yields any sort of conclusive answer, please take into account:

  1. Josephus may have rearranged some of the episodes in his history. Like many ancient writers, he has a flexible approach to chronology. In this case, Theudas's rebellion may well have preceded Acts 5 (which is to be dated to the 30s).
  2. It was a common enough name that there could have been two troublemakers named Theudas (Corpus Inscriptionum Graecarum 2684, 3563, 3920, 5698). As Witherington notes, Josephus "mentions four Simons within 40 years and 3 Judases within ten years as instigators of rebellion!" (The Acts of the Apostles: A Socio-Rhetorical Commentary [Grand Rapids: Eerdmans, 1998], 239). Names often repeat.
  3. Luke could have made a mistake. That is, he may have been faithful to the findings of his research, but misinformed.

Was Luke wrong (3)? Given his remarkable accuracy on things geographical and historical, I am reluctant to settle on this possibility -- my bias -- though it's a possibility all the same. I suspect that either (1) or (2) is the case. Wherever you land, Christians should have a high regard for truth, history, and verification of facts -- which is why your question is welcome.