Two Stages of Dissemination
The ancients knew of (at least) two very distinct steps in the dissemination process. One first made notes or memoirs; then one crafted, if desired, those notes or memoirs into a polished rendition for official publication:
Suetonius, Life of Julius Caesar 56: He left memoirs [commentarios] too of his accomplishments in the Gallic and in the Pompeian civil war, for the author of the Alexandrian, African, and Spanish wars is uncertain. Some suppose that it was Oppius, others Hirtius, who also supplied the most recent and unfinished book of the Gallic war. To the memoirs [commentariis] of Caesar in the Brutus itself Cicero refers thus: "He wrote memoirs [commentarios] to be strongly commended indeed. They are naked, straightforward and lovely, stripped of the vesture of every adornment of oration; but while he wished others to have these things prepared, whence those who wished to write a history might assume, he ended up gratifying the inept, who wish to use the curling irons on them. Sane men, in fact, he deters from writing." Of these same memoirs Hirtius thus proclaims: "They are so approved in the judgment of all men as to have taken opportunity away from writers, rather than to have offered them one. Our admiration for his accomplishment, nevertheless, is greater than that of the rest; for they know how well and faultlessly, and we also how easily and quickly he wrote them out." Asinius Pollio supposes that they were composed with too little diligence and integrating too little truth; since Caesar for the most part both believed the things related through others and improperly published things related through him, either deliberately or perhaps from memory lapse; and he thinks that he was going to rewrite and correct them.
Galen, Concerning His Own Books, prologue: Why the many read my [books] as their own, you yourself know the reason, most excellent [κράτιστε] Bassus. For they were given to friends and disciples without inscription [χωρίς ἐπιγραφής], as nothing was for publication [οὐδὲν πρὸς ἔκδοσιν], but were made for those who requested [δεηθεῖσιν] to have notes [ὑπομνήματα] of what they heard. So, when some of them died, those with them who had them and were pleased [ἀρεσθέντες] with them began to read [ἀναγινῶσκον] them as their own. [....] ...having shared [κοινωνησάντων] them traveled to their own fatherland and, after passing some time, some here and others there began to make them into lectures [ἐπιδείξεις]. In time, after they were all exposed, many inscribed [ἐπεγράψαντο] my name on the repossessed [text]. And, having found that they differed from all the others, they carried them to me, encouraging me to rectify them. So since, as I said, they were not for publication [οὐ πρὸς ἔκδοσιν], but were according to the habit and the need [ἔξιν τε καὶ χρείαν] of those who requested [τῶν δεηθέντων] them, it was likely at any rate that some be stretched out and others pressed together, and the interpretation [ἑρμενείαν] and teaching [διδασκαλίαν] of the theorems should be either complete [τελείαν] or lacking [ἐλλιπή]. It was clear, at any rate, that those written from the things that were spoken [τοῖς εἰρημένοις] would not have the completion of the teaching, nor would have been examined accurately [διηκριβωμένον], as they neither requested [δεομένων] nor were able to learn [μανθάνειν] all things accurately [ἀκριβώς] before having some habit [ἔξιν] in the essentials. These kinds of books [βιβλία] some who came before me wrote up as outlines [ὑποτυπώσεις], just as some wrote sketches [ὑπογραφάς]. And others wrote introductions [εἰσαγωγάς] or synopses [συνόψεις] or guides [ὑφηγήσεις].
It is clear that, while the notes or memoirs stage was considered a normal step in arriving at a polished product for publication, there was not always a perceived necessity to push the text beyond that first stage, especially in a didactic situation such as that described by Galen.
Multiple Editions
Sometimes an author, upon publishing (in whatever sense) a work, might republish it:
Obviously, if a person can hijack an edition of a work, then it seems unlikely that any edition of that work can have truly been withdrawn from circulation in antiquity. Multiple editions of the same work in circulation might be a good source for textual variants.
Bootlegged Editions
The extract from Tertullian above gives an example of bootlegging, as does the following:
Any situation involving oral teaching would be susceptible to this kind of treatment at the hands of listeners, whether well meaning or not.
Additions & Subtractions
We have rather many indications from antiquity that books were regularly subtracted from or added to (interpolated):
Artemidorus, Oneirocritica 2.70: 70 I ask those who read my books not to add to or remove anything from the present contents. For any person who is able to add points to my work would more easily write a work of his own. And if certain things that I have written in these books seem superfluous, the reader should use only those things that please him without discarding the rest of the books. For he should realize that it was out of obedience to Apollo, the overseer god and guardian of all things in addition to being my own native god, that I undertook this treatise. Apollo has encouraged me in the past, and now especially, when I have made your acquaintance, he clearly presides over my work, and has all but commanded me to compose this work.
Eusebius, Church History 4.23.12: 12 The same writer [Dionysus of Corinth] also speaks as follows concerning his own epistles, alleging that they had been mutilated: "As the brethren desired me to write epistles, I wrote. And these epistles the apostles of the devil have filled with tares, cutting out some things and adding others. For them a woe is reserved. It is, therefore, not to be wondered at if some have attempted to adulterate the Lord's writings also, since they have formed designs even against writings which are of less account." There is extant, in addition to these, another epistle of Dionysius, written to Chrysophora, a most faithful sister. In it he writes what is suitable, and imparts to her also the proper spiritual food. So much concerning Dionysius.
Rufinus, preface to On the Falsification of the Books of Origen: I do not think it can be doubted that it could in any way happen that a man of such an education and so prudent — which of course even his accusers can grant — a man who was neither foolish nor insane, would have written what is contrary to himself and repugnant to his own opinions. Or even if we grant that this could in some way have happened — for perhaps someone will say that in the decline of life he might have forgotten what he had written in his youth, and that he later brought forth things at variance with what he had formerly thought — what shall we do about the fact that sometimes in the very same passages, and, so to speak, in practically the next section, an opinion is found inserted that is of a contradictory sense? Could he have forgotten his own views in the same chapter of the same book, [or] sometimes, as we have said, immediately in the next section? For example, when he had said just before that nowhere in all the Scripture is it found that the Holy Spirit was said to have been made or created, would he immediately add that the Holy Spirit had been made along with the rest of the creatures? Or again, could he who has pointed out that the Father and the Son are of one substance — which is said in Greek as ὁμοούσιος — have said in the immediately subsequent sections that he is of another substance and was created, the one whom he had but a little before declared to have been born of the very nature of God the Father? Or again, concerning the resurrection of the flesh, was it possible that he who so clearly declared that the nature of the flesh ascended with the Word of God into heaven, and there appeared to the heavenly powers, presenting to them a new and marvelous sight of himself, has said, on the other hand, that this [flesh] is not to be saved? Since, then, these things could not happen even to a man who was out of his mind and who was not sound in the brain, I will briefly clarify the cause of this to the best of my ability.
We also have rather many indications that entire books could be forged under false names, but that topic seems to deserve a treatment unto itself. Bart Ehrman has published books dealing with ancient forgery of books.
I conclude (for now) with a quote from a modern researcher on the topic at hand:
It is my feeling that the hazards of ancient book publication may have particular implications for how early Christian and Jewish texts have come down to us.
Ben.