ייִדישער מחשבֿה אויף קהלת 9:18

Of the ninth degree are those who believe in the Torah and in reward and punishment in both worlds. In serving G-d, their intent is to do so for His Name's sake and in the way befitting Him alone, but they are not careful to avoid whatever is detrimental to this service, and when detriments have caused injury they do not know from where the trouble came. This is suggested in the verse: "Dead flies make the apothecary's oil to ferment and send forth a foul odor; so a little folly outweighs the rarest wisdom and honor" (Koheles 10:1). Further, "One sinner destroys much good" (ibid. 9:18).