Working through the 11th century Greek Gospels Lectionary 252, I came across an interesting variant in John 3:9-10. In L252, Nicodemus rebukes Jesus for his ignorance of the regenerating work of the Spirit and then proceeds to instruct him with regard to his messianic role. The lectionary lacks πως δυναται ταυτα γενεσθαι; απεκριθη Ιησους και ειπεν αυτω in vv. 9-10, leaving a text that reads:
"Nicodemus answered and said to him, 'Are you the teacher of Israel, and you do not understand these things? Truly, truly, I say to you...'"
This shorter, more theologically primitive reading is clearly the initial text of the Gospel, which has been replaced throughout the entire rest of the manuscript tradition. This process of intentional supression of heterodox readings is verified within the manuscript itself by the secondary insertion of the lacking text. The original teaching of the Gospel of John is that Christ was theologically misinformed and needed to be taught by the Jewish religious leaders why he was here...
Alternatively, the text was accidentally dropped out by homoioteleuton (και ειπεν αυτω ∩ και ειπεν αυτω) in this medieval lectionary, but I doubt I could even get a single article out of that... ;)