"Therefore they have laid their hands boldly upon the Divine Scriptures, alleging that they have corrected them. That I am not speaking falsely of them in this matter, whoever wishes may learn. For if anyone will collect their respective copies, and compare them one with another, he will find that they differ greatly. Those of Asclepiodotus, for example, do not agree with those of Theodotus. And many of these can be obtained, because their disciples have assiduously written the corrections, as they call them, that is the corruptions, of each of them. Again, those of Hermophilus do not agree with these, and those of Apollonides are not consistent with themselves. For you can compare those prepared by them at an earlier date with those which they corrupted later, and you will find them widely different. . . . For they cannot deny the commission of the crime, since the copies have been written by their own hands. For they did not receive such Scriptures from their instructors, nor can they produce any copies from which they were transcribed." (Eusebius Hist. eccl. 5.28, NPNF2)This story brings to light a few details that help shed light on copying practices in the late second and early third centuries;
1) Scribes, or rather copyists (they don't appear to be professional scribes), DID alter the text in order to make the Bible better support their theological perspective.
2) Because literary documents were circulated through social contacts (see previous post here), it was impossible for major alterations of a text, in this case the New Testament, to go unnoticed by others in this circle. Scholars, or even your average reader, could compare their copies with those being read in another Christian circle (in this case adoptionist).
3) Eusebius implies that copies of these altered texts are readily available for anyone to examine. Perhaps they are in the library of Cesarea (for comparison purposes).