I was recently discussing with my son the chiddush of the Nesivos (234) that violating an issur derabbanan b’shogeg does not require kapparah. The logic here (see Sha’arei Yosher 1:7) is that what the Torah prohibits through “lo tasur” is rebelling against the authority of Chazal. If one is not aware that something is prohibited, then by definition one is not rebelling – you can’t rebel against something you are not even aware of. (This assumes that the Nesivos means that a violation of a derabbanan b’shogeg is not a ma’aseh aveira, though technically speaking all the Nesivos says is that no kapparah is needed.)
The Kli Chemdah on this week’s parsha has an even bigger chidush. He writes that violating a lav she’ain bo ma’aseh b’shogeg is not a ma’aseh aveira. A lav can be violated only through either deed or thought. By definition, a person who is shogeg has not made a conscious decision not to take action – he/she is simply unaware that something needs to be done.
One of his proofs comes from a diyuk in Rashi. Rashi (Brachos 2a) writes that the seyag of not eating korbanos past chatzos was instituted lest a person inadvertently continue eating even past daybreak when the meat becomes nosar. Why does Rashi need to introduce eating into the equation – leaving the meat over itself violates “lo tosiru ad boker?” (Tzelach) The Kli Chemdah answers that the Chachamim would never have instituted a seyag just to prevent leaving over the korban meat. Since nosar is a lav she’ain bo ma’aseh, forgetting or inadvertently not consuming the meat before daybreak is not a ma’aseh aveira; it involves neither intent, thought, nor action. The seyag was invoked only to prevent eating, which would entail action and therefore constitute a ma’aseh aveira.