Yesterday we raised the question of why there is no concept of chezkas yom, a status quo of it being day, which would resolve the safeik status of bein hashemashos. I think at least three answers are possible:
1) As discussed yesterday, there cannot be a chazakah of it being day and not night because by definition that situation will change, and a chazakah cannot be established based on an unstable situation (chazakah he'asuya l'hishtanos - see Tos Gittin 2b).
2) The Brisker answer which was very popular in the comments is that bein hashemashos is not a safeik in the same sense that a piece of meat which may be cheilev or may be shuman is a safeik. In the latter case, the meat is either cheilev or shuman, but we cannot determine which. In the case of bein hashemashos, the period of twilight is by definition a mixture of both day and night, an indeterminate combination of both.
3) One can also answer with a Rogatchover-ish sevara. Bein hashemashos is not a single block of time, but is really an infinite number of constantly divisible smaller units of time. Chazakah works when we speak of maintaining the status quo of an object, but the status quo on one object is not transferable to another. Since each moment of bein hashemashos is an independent unit / object, chazakah cannot resolve a safeik in units of time (compare with this post).
So much for R' Scheinberg's question -- now for my question (I think this one is a little more challenging). If someone mistakingly followes an errant ruling of Beis Din, he/she is not liable for his/her own korban. The gemara in Horiyos discusses a case where Beis Din made an incorrect ruling but then retracted. What if someone followed the original ruling in error -- is that person liable for his/her own korban or not? Was the person aware of the retraction and acting of his/her own volition, or can they pass of their mistake as stemming from Beis Din's error?
Sumchus opines that the person is exempt from korban and he offers an analogy (see Horiyos 4a; I am going to simplify the case a bit for clarity): someone offered a korban bein hashemashos creating a safeik whether the korban was brought during the day and is kosher or whether it was offered at night and is pasul -- we assume the korban was brought by day and is kosher. So too, had a person acted before Beis Din retracted he/she would be exempt from korban; therefore, we assume the person's error was caused by Beis Din's ruling and he/she remains exempt until proven otherwise.
The Tosfos Rosh on the page explains the analogy a little better -- just like in the bein hashemashos case where we are unsure if the korban was offered at day or night we assume the korban was offered by day because "mukminan hayom b'chezkaso", there is a chazakah / status-quo of day which exists until proven otherwise, so too in the case of the potential exemption based on the ruling of Beis Din we assume there is a chazakah status-quo of the person being exempt until proven otherwise.
Question: we gave three beautiful sevaros to explain why there is no such thing as a chazakas yom that can resolve the status of bein hashemashos. Yet, here the Tosfos Rosh tells us that "mukminan hayom b'chezkaso", there is a concept of chezkas yom which we can use to certify as valid a korban offered bein hashemashos. (And note that the Tos Rosh specifically writes "chezkas hayom" as opposed to the next sentence discussing the Horiyos case where he refers to the "chezkas hagavra".) How can that be??? What is the difference between our original qustion and this case of Sumchus?
Hint: the only answer I can think of so far involves using R' Shimon Shkop-ish reasoning. Waiting to see if someone has a nice Brisker answer for this one!