When we had to work through the whole of the New Testament in a more systematic way, we started with the Pauline corpus. The assumption was that the letters of Paul did not pose as many problems as some other parts of the NT, and this assumption bore out. Apparently there is something in tightly argued prose that puts it in less danger of textual change than simple narrative, especially narrative with synoptic parallels. Yet even within the Pauline corpus the same phenomena are present that you can find in the Gospels. Ephesians and Colossians contain sufficient parallel material to allow for cross-contamination, and the same happens with Galatians and Romans.
However, influence from parallel passages is not limited to similar sentences or similar narratives. There are all sorts of phenomena that can spark off cross-contamination. And, true to the reputation that the Greek-Latin manuscripts have, a number of these are found in the D(06) F(010) G(012) cluster. Two obvious examples to illustrate the point.
Gal 4:17 ζηλοῦσιν ὑμᾶς οὐ καλῶς, ἀλλὰ ἐκκλεῖσαι ὑμᾶς θέλουσιν, ἵνα αὐτοὺς ζηλοῦτε. (‘They are not zealous for you in a good way, but they want to shut us out so that you may be zealous for them’)
Paul finishes the sentence with ζηλοῦτε and after a negative sentence such as this one often introduces a positive contrast, which more or less follows in the next verse. Somehow, however, linked by the contrasting pattern and specifically the link work ζηλοῦτε we find in the D(06) cluster the extra words ζηλοῦτε δὲ τὰ κρείττω χαρίσματα (‘but be zealous for the better gifts’). These words are a clear echo of 1 Cor 12:31, though with some minute differences. Is this addition simply a marginal note that slipped into the main text? Is it the result of someone who is copying Galatians from memory more than from a document? Who knows, but the extra words are there now. The link is tenuous but we could reconstruct the triggers, and therefore learn something about the way in which copying can be affected.
The second example is just as gorgeous and concerns influence from within Galatians.
Gal 3:1: Ὦ ἀνόητοι Γαλάται, τίς ὑμᾶς ἐβάσκανεν (‘O foolish Galatians, who has bewitched you’).
The D(06) cluster, now with a whole lot of additional support, adds the words τῇ ἀληθείᾳ μὴ πείθεσθαι (‘to not obey the truth’). The source of these words is Gal 5:7 and the trigger here is the start of the actual question, τίς ὑμᾶς, followed by a verb (ἐβάσκανεν and ἐνέκοψεν), and both in an accusatory question. Not that much to go on, mainly the τίς ὑμᾶς part. Still it is enough of a trigger to import wording from elsewhere.
Both Galatian cases are quite clear to me (the second of course not for those who favour a Byzantine text, though I assume they would acknowledge the mechanism in the first case). Which leads me to the conviction that if this mechanism is at work in clear instances, it might well be at work in many less obvious cases. Therefore, if there is an explanation available that can explain the longer text as being the result of influence from elsewhere, the shorter reading has a strong transcriptional case.