Yet it goes beyond this. I have always wondered why Epiphanius attributes some of the stuff referenced in our copies of Against Heresies as characteristic of the Marcosians and identifies them as belonging to another Valentinian subgroup. Indeed are the Marcosians really a Valentinian subgroup or is that inference drawn from the position they assume within the current ordering of Book One? In other words, if the assembling of heretical groups wasn't conducted by Irenaeus but by a later editor or as an 'afterthought' by Irenaeus himself we might be more careful about assuming too much with regards to the ordering of sects in the text.
Indeed most people assume that the list of heresies that starts with Simon Magus and ends with the Cainites represents an incorporation of Justin's Syntagma into Irenaeus's original work. Who was responsible for this incorporation of 'foreign material' into the work? Does it have to be Irenaeus? Moreover is it possible that 'the Exposure and Overthrow of Knowledge falsely so-called' was in fact a separate work which only became 'Against Heresies' because of a later effort to collect all - or most - of Irenaeus's anti-heretical writings into a five volume collection (a la the Five Books Against Marcion)?
A strong clue in support of this idea (i.e. that 'the Exposure and Overthrow of Knowledge falsely so-called' was originally a work limited to the Valentians = Tertullian's Against the Valentinians) comes from another work of Irenaeus was which discovered relatively recently - the Proof of the Apostolic Preaching. This work had long been thought to be lost and in fact was only discovered in the twentieth century long after much of the work to understand 'Against Heresies' had already been established. I think if we look carefully at the closing words of Apostolic Preaching show clearly that the work was originally conceived as a rejection of the Valentinian faith. Just look at how Irenaeus excludes two other type of heresies from his description of the work:
So that none should imagine God the Father to be other than our Creator, as the heretics imagine; (for) they despise the God who is, and make gods of that which is not; and they fashion a Father of their own above our Creator, and imagine that they have found out for themselves something greater than the truth. For all these are impious and blasphemers against their Creator and against the Father, as we have shown in the Exposure and Overthrow of Knowledge falsely so-called. And others again reject the coming of the Son of God and the dispensation of His incarnation, which the apostles delivered and the prophets declared beforehand, even such as should be the summing up of mankind, as we have shown you in brief: and such also are reckoned amongst those who are lacking in faith. And others receive not the gifts of the Holy Spirit, and cast away from themselves the prophetic grace, watered whereby man bears the fruit of life unto God: and these are they of whom Isaiah speaks: For they shall be, saith he, as an oak that is stripped of leaves, and as a garden that hath no water.275 And such are in no wise serviceable to God, seeing that they cannot bear any fruit. So then in respect of the three points276 of our seal error has strayed widely from the truth. For either they reject the Father, or they accept not the Son and speak against the dispensation of His incarnation; or else they receive not the Spirit, that is, they reject prophecy. And of all such must we beware, and shun their ways, if in very truth we desire to be well-pleasing to God and to attain the redemption that is from Him.
The idea that the Marcionites and some other heretical group were excluded from the contents of the Exposure and Overthrow of Knowledge falsely so-called clearly implies that the original work was quite different from what we presently possess as 'Against Heresies.' More on that later (my son is driving me nuts right now with demands I play superheroes with him).
