When the three friends of Job spoke to him, disagreeing with him, were their homilies inerrant? No, but they were written through inspiration and the content was recorded inerrantly. But God did not mean us to live by their faulty doctrines. To go even further, what Job himself said was sometimes mistaken, but was recorded by inspiration of God, and inerrantly portrayed in Scripture.
Likewise, what Stephen said was recorded by inspiration of God and recorded inerrantly. God never guaranteed to Stephen that what Stephen would not make errors of history. (But of course we can live by what he preached because he preached Christ and Him crucified.) However, when the Bible does record history (as in Acts), that history is inerrant, since it is given as fact and not as the record of someone's sermon.
And again, we are not allowed to change Greek grammar by using English grammar, and we are not allowed to rearrange Greek grammar to fit our presupposition of inerrancy. I believe and stand for an inerrant Bible, but I have to take it as it is, difficulties and all.