Jamieson, Fausset and Brown on Acts 23:5

5. I wist not ... that he was the high priest--All sorts of explanations of this have been given. The high priesthood was in a state of great confusion and constant change at this time (as appears from Josephus), and the apostle's long absence from Jerusalem, and perhaps the manner in which he was habited or the seat he occupied, with other circumstances to us unknown, may account for such a speech. But if he was thrown off his guard by an insult which touched him to the quick, "what can surpass the grace with which he recovered his self-possession, and the frankness with which he acknowledged his error? If his conduct in yielding to the momentary impulse was not that of Christ Himself under a similar provocation (Joh 18:22, 23), certainly the manner in which he atoned for his fault was Christ-like" [Hacket].