The gemara discusses someone who manages to transgress all 39 melachot of shabbat without knowing that he is transgressing them, all the while knowing that it is actually shabbat.

Tosfot asks why we don't assume that he thought they were all an asseh ("V'yom Hashvii Tishbot") instead of a lo ta'aseh.

Someone had a seemingly better question: Why not just say that he knew that it was shabbat because of the aseh to make kiddush hayom? This way we all know that he knew that it was shabbat, and since he didn't know any of the other halachot, he was able to transgress all 39 melachot.