Niddah 56 - Creepy-crawly in an Alley
The Torah lists eight crawling animals that convey ritual impurity when they are dead . If such a dead crawler was found in an alley, then ritually pure objects in this alley are declared retroactively impure: perhaps the crawler that is found now could have been there for several days, and could have touched the pure object. However, if someone can make a statement, "I checked this alley on that day, and it was free of such crawling animals," then the retroactive impurity stops and does not go beyond this day. Also, if someone swept the alley, then there is no suspicion that the animal was there before: we have to assume that he swept away all impurities, and it could only appear there after sweeping.
In a similar vein, a bloodstain on a tunic renders ritually pure objects impure back in time, but only till the moment that one says, "I checked this tunic, and it did not have a bloodstain on it," or until the last time it was laundered.
Art: Kauffmann, Angelica - Julia, wife of Pompey, faints at the sight of his bloodstain...