He argued that one can be obligated a sin offering for מתעסק [a mistake in the facts,] only when there is הנאה pleasure.. That is the normal case of a sin offering is when there is a mistake in law, not in the physical facts.

I saw that I had written something in my notes on Shas (Gemara. That is the Oral Law) in the name of my learning partner. He argued that one can be obligated a sin offering for מתעסק [a mistake in the facts,] only when there is הנאה pleasure.. That is the normal case of a sin offering is when there is  a mistake in law, not in the physical facts.
Later I saw that this can not be the case to the Rambam. Rav Shach goes into this in laws of איסורי ביאה א:י''ב. What my learning partner suggested is in fact the opinion of Tosphot and all other rishonim but not the Rambam.
To make things short:
The Rambam in three places says the reason מתעסק בשבת is not obligated in a sin offering is because מלאכת מחשבת אסרה תורה. He does not say is is because he was not נהנה (get pleasure.) ה' שגגות פרק ב' ה''ז פרק ז' הי''א and also in פירוש המשנה כריתות פרק ספק אכל in all three places the rambam says the reason מתעסק בשבת is פטור  is because מלאכת מחשבת אסרה תורה and he says nothing about whether he was נהנה or not.
However נהנה even to the Rambam can make one a מזיד in order to be obligated in lashes.
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