Dirk Jongkind reflects on harmonization triggers, especially in the Pauline corpus. In part, he suggests,
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.
For the balance of Jongkind’s comments, see his original post on Tyndale House’s Greek New Testament blog. See also the cross-post and further discussion on Evangelical Textual Criticism. For more information on Tyndale’s Greek Testament project, see also Tyndale House GNT and the in-post related links.