Aside from mistranslating Ioudaios as "Jew" in the New Testament, the most pervasive scholarly mistake is probably Luther's trap in Rom 14:1-15:6, defined by Mark Nanos:
"Luther recognized that Paul was clearly instructing the 'strong' not to judge the opinions of the 'weak'; however, Luther tripped into the very trap of judging them and then read this judgment as Paul's. That is, Luther was tripped by the faulty assumption (1), that the 'weak' were Christian Jews, into the trap of assumption (2) with its inherent, inescapable contradiction wherein he indulges in the very same kind of judging Paul warned the 'strong' (which Luther considered himself) to avoid." (The Mystery of Romans, p 92)Nanos then proceeds to demonstrate why the "weak in faith" in Rome were non-Christian Judeans rather than Christian Judeans.
*Note: From this day foreward, in the wake of Elliott's essay, I will be religiously using "Judean" instead of "Jew", the latter only when citing others (as above).