Chullin 95a: If there are nine stores that sell kosher meat and one that sells non-kosher meat, and a person bought from one of them but does not know which one, it is forbidden.
תא שמע: תשע חנויות כולן מוכרות בשר שחוטה, ואחת מוכרת בשר נבלה, ולקח מאחת מהן, ואינו יודע מאיזה מהן לקח ־ ספקו אסור.
יו”ד סימן ק”י סעיף ה: מי שלקח בשר מהמקולין ואפילו חתיכה הראויה להתכבד ונמצאת טרפה במקולין ולא נודעו חתיכות הטרפה ואינו יודע מאיזו לקח, כל מה שלקחו מהמקולין קודם שנמצאת הטרפה מותר שלא נפל הספק בקבוע אלא לאחר שפירש וכיון שהרוב כשר מותר, אבל ליקח מכאן ואילך אסור ואפילו חתיכה שאינה ראויה להתכבד שאין הכל בקיאין בזה ויטעו בין ראויה להתכבד לשאינה ראויה.
Shortly before Pesach 2013, the owner of Doheny Glatt Kosher Meats in Los Angeles was caught on camera instructing his employees to bring boxes of unsupervised meat into his store. The video, shot by a private investigator, showed the owner waiting until the mashgiach left the premises and then having the boxes carried in from the back of his SUV.
On Sunday, the 13th of Nisan (March 24, 2013), staff members from the Rabbinical Council of California (RCC) as well as a handful of other rabbis and lay leaders from the Orthodox community gathered to watch the incriminating video in the office of Rabbi Kalman Topp, rav of Beth Jacob, the largest Orthodox synagogue in Pico-Robertson. Also present were Rabbi Elazar Muskin of Young Israel of Century City and Rabbi Yosef Kanefsky of B’nai David-Judea. After watching, the rabbis called in the store owner, who admitted that he brought in the unsupervised meat, although he claimed it was kosher.
Based on this information, the RCC revoked its certification from Doheny effective on March 24 at 3 PM. They stated, however, that any meat bought from the store before 3 PM on that day was considered kosher. Rabbi Yisroel Belsky was consulted and he also agreed with this ruling.
The reason is based on the rule that when it comes to kavua d’rabanan, there is no kavua lemafrea.
To explain: our Gemara is talking about a case when the non-kosher store was recognizable as such all along. This is kavua d’oraisa. If later on it became recognizable as treif, but it was not recognizable at the time of purchase, there is a dispute between the Ran and the Rashba as to whether the stringency of kavua applies retroactively. We pasken like the Rashba that it does apply. So, for example, let’s say the investigation had revealed that ALL the meat at Doheny was treif. A sign was promptly hung in the store window at 3:00 PM saying that the hashgacha was removed. Then someone came to a rav and asked, “I bought this meat at 2:00 PM and I’m not sure if I bought it from Doheny or one of the other 9 stores in Los Angeles.” The rav would tell him it is forbidden because it is kavua lemafrea.
However, in our case not all the meat in the store was treif. It was only 8 boxes of questionable meat out of about 300 coming into the store that day. There is no kavua d’oraisa, even retroactively, because it was never recognizable, even later, which pieces of meat were treif. But what about kavua d’rabanan? The Rabbinic rule that “a piece fit to serve to an honored guest” does not become nullified in a mixture is called kavua d’rabanan (see Zevachim 73a and 73b, and Tosafos on 73b s.v. ela), because it follows similar rules to those of kavua d’oraisa: you are not allowed to take directly from the mixture, but if somehow a piece got separated from the mixture on its own, you are allowed to eat it. Does this rule also operate retroactively? The Shulchan Aruch (110:5) rules that it does not. Thus if Doheny was selling hundreds of nice pieces of meat fit for a guest, and it was discovered at 3:00 that a small fraction of them were treif, those pieces purchased before the discovery are not forbidden. After the discovery, all the meat in the store is forbidden, even the ground and chopped meat, because people might confuse it with the big pieces fit for a guest. )