Irenaeus makes an intriguing comment when he is about to discuss the variant readings of the number of the beast (666 vs. 616) in Rev 13:18. I quote from the translation in the Ante-Nicene Fathers, Against Heresies 5.30:
"... and this number being found in all the most approved and ancient copies [of the Apocalypse], and those men who saw John face to face bearing their testimony [to it] ..."
Besides the testimony of the 'ancient copies' (which in Irenaeus's case cannot have been very ancient), Irenaeus also appeals to the testimony of those who saw John face to face and who apparently confirm the reading 666. Though their testimony might have come to Irenaeus by means of a written testimony (I don't know), it is fascinating that he attributes independent force to the wider tradition within the church which secured this particular reading. I have never seen this appeal to 'oral testimony' before in textual criticism of the NT, but am wondering if there is more out there.