Not to be all Prof. History over here (except that it is actually my job, so, whatever) but the whole ‘dog returning to its vomit’ thing is a mainstay of medieval Inquisitors’ handbooks and anti-Jewish legislation, where vomit = heresy. Often the dog is implied. Two examples, because of the motto we say at historians’ parties (there are no parties) is: Always Be Providing Evidence To Back Up Your Assertions.

(1) From the seventh century Visigothic law code called the Visigothic Code (awesome title, very original): “Every Jew who renounces his former errors, and is converted to the holy faith of Christ, shall inscribe in his confession the Christian symbol; shall promise that he will under no circumstances return to his errors, as he would to his vomit; and shall profess that henceforth he will, in compliance with the terms of his written confession, in no way change anything which we have included in this law.”

(2) From the trial of Joan of Arc (1431): “But the fire of her [Joan's] pride, which had seemed quenched, was revived by demonic winds and enkindled into destructive flames, and the miserable woman returned to her errors and lying follies that she had earlier vomited forth. Finally, as ecclesiastical sanctions dictate, to prevent her from infecting other members of Christ, she was handed over to the judgment of secular authority, which determined that her body should be consumed by fire.” (D. Hobbins, ed., The Trial of Joan of Arc (Harvard Univ. Press, 2005), pp. 210-211.)

I had to write a You Can Make It Up list that was This List + Damon Weaver, because that kid is awesome, and there is no question that his mad President-interviewing skills made him one of the Best People of 2009. I said good day, sir!