At the start of Shabbos 69b, we have 'ka mashma lan' implying, per the Artscroll, that someone unaware of the chatat sacrifice for shogeg in false oaths, doesn't suffice to make someone obligated to a sacrifice. So it seems they're off scott free, even though they knew that it was prohibited to violate an oath.
I take that to imply the following odd situation I'd like help to resolve: If the above person was aware of prohibition but shogeg re: sacrifice and they got off w/o even a sacrifice (so kal vachomer not a worse penalty). But then if someone was shogeg on the prohibition too, it sounds like they DO bring a penalty. Yet they are more innocent!
Why should no degree of fault incur a penalty when some degree of fault does not??
Yaakov Saibil
The Chatas sacrifice is not a penalty. It is a method of atonement.
If one sins b'Shogeg - i.e. due to lack of knowledge - he is eligible to atone for his sin by bringing a sacrifice. But that is not so if he carries a greater degree of fault because he sinned either with full or with partial intent (such as the case you described above, where he was aware of the prohibition but not the sacrifice). In such cases a person is not eligible for atonement through bringing a sacrifice. (See Makos 2b, "Meizid does not deserve a Kaparah...") So he is not exactly going off "scott free", as you put it.
Best wishes,
Mordecai Kornfeld
Kollel Iyun Hadaf