[This subject is part of a large Tosphot in Yoma. I just wanted to bring up one little point that I think is interesting.]
This approach of R. Yehuda seems to be contrary to the way the Rambam understands Rabbinical law.
To the Rambam, the Torah gives permission to the Sanhedrin in Jerusalem (or people with what the Torah considers ordination--not the ordination that is common today) to interpret the law based on the thirteen principles of how to derive laws from verses and also to make a fence around prohibitions. [Also, they had some traditions of what is considered work on Shabat and other oral traditions that they have to hand down.] This we see in the verse in Deuteronomy "you shall listen to all they teach you." [The whole verse says basically when you have cases in Torah Law that come up, and you can't decide, you shall go to the supreme court in Jerusalem and follow all that they teach you.]
The Rambam makes it clear that it is not up to every individual to decided how to interpret the the law. This is clearly stated in the Torah itself.
But the law to listen to the Sanhedrin is specially a law to not ignore them. And this can't apply to an unintended act. When one forgets the law he is not ignoring anything.
The resolution to this is that the Rambam in fact decided in a "thing not intended" like R. Shimon and not like R. Yehuda.
I hope it is clear what I am trying to say. We have two arguments between R. Yehuda and R. Shimon. One about work not intended. The other about work done for a different than purpose the work was done in the tabernacle in the desert. The question here is on R Yehuda. If we understand a rabbinical decree in the way the Rambam does, then how does the opinion of R. Yehuda make sense? That is the question I intend to answer in this short essay.
The big issue I have not addressed here is if there is in fact any authority to make extra decrees not in the Torah and from the Commentary on Pirkei Avot from the amoraim it seems there is no such authority. This book is called אבות דר' נתן and it is included in every edition of the Vilna Shas. The basic idea there i on the Mishna "Make a fence around the Torah" and the general approach there is to say that Adam HaRishon added to the command of God [don't eat and do not touch] and that caused him to fall. R.Yose said there "Better ten hand-breaths high that stand rather than 100 yards high that fall." There the Gra makes a few corrections to the text. I showed this to Rav Eliyahu Silverman the Rosh Yeshiva of the Yeshiva that goes by the path of the Gra in the Old City of Jerusalem and he agreed with me that that is the meaning of that commentary on the Mishna