Reference for Megillah 16:11

אלא טהור השתא מלטמא כלי חרס בהיסט אע"ג דהדר חזי לא מטמא למפרע

Rather, it means that he is clean now so far as not to defile earthenware vessels by moving them; and even if he observes [an issue] again, he does not defile them retrospectively.

Jastrow