This is concerning Apollos and why Luke makes no mention of Priscilla and Aquila baptizing him.
It is possible, as Bruce points out, that they might have thought it unnecessary, since he already had the Holy Spirit. It may also be that they thought that John’s baptism was valid as foreshadowing of Christian baptism, and perhaps Apollos had undergone that baptism. It is interesting that we have no evidence that the Twelve ever received Christian baptism, and it is even more interesting that Paul not only seems to imply that not all of his converts were baptized but also that he was glad, under the circumstances in Corinth, that this was the case (see 1 Cor. 1:14–17). All of this cautions us against assuming that later beliefs about the necessity of Christian baptism, even its necessity for salvation, were shared by all or perhaps even most early Christians.
Witherington, B., III. (1998). The Acts of the Apostles: A socio-rhetorical commentary (567). Grand Rapids, MI: Wm. B. Eerdmans Publishing Co.