I would like to suggest a proof from the well-known opinion of the Nesivos ha'Mishpat (Choshen Mishpat 234:3), that if somebody ate the meat b'Shogeg it is not considered that he has "broken down the fence."

1) The Nesivos ha'Mishpat writes that while if someone ate food that is forbidden by the Torah, even inadvertently, he requires atonement and must do Teshuvah to save himself from suffering. In contrast, if he transgressed a Rabbinical prohibition, he requires no atonement and it is as if he has violated nothing.

2) The Nesivos cites a proof from the Gemara in Eruvin (67b) where Rav Yehudah said that concerning a Torah prohibition first one asks questions and only afterwards puts it into practice. In contrast, concerning a Rabbinical prohibition one does the action first and only asks questions later. Rashi explains that the scenario is that a Chacham issues a permissive ruling on a Torah matter. One of the students challenges the Psak. He must first ask his question to save us from transgressing a Torah prohibition. However, if the Heter that the Chacham gave involved a Rabbinical issue, we let him put his ruling into action before discussing it.

3) The Nesivos proves from here that there is no punishment for transgressing a Rabbinical prohibition b'Shogeg, because, otherwise, we would not let one transgress and receive a punishment. It must be that if someone inadvertently ate food which is forbidden by the Rabanan, it is as if he has eaten Kosher food.

4) I argue that according to Nesivos, if someone ate meat b'Shogeg in the Nine Days he will not receive the punishment that one receives for breaking down the fence of Chazal.

5) Admittedly, there are opinions who differ with the Nesivos, but the Nesivos is a major Shitah.

Kol Tuv,

Dovid Bloom