Pretty much just that.
(If a person is misasek when doing an aveira, he is not liable for anything. If, however, he is shogeg, then he often has to bring a korbon. ‘Misasek’ is not a lack of awareness about the halacha, but rather means when a person makes a mistake about the situational factors surrounding his act, those factors in fact causing the act to be one which is forbidden. Thus, as a result, the person does something forbidden without realising that it was forbidden. Shogeg means when someone forgets or does not know that the action he's doing is in fact forbidden - but he is fully aware of the situational factors; he knows exactly what he is doing, he just isn’t aware it’s wrong.)
When a person forgets it’s Shabbos, isn’t that a lack of situational awareness? We’re not talking about a case where the person knows its Saturday but forgot that on Saturday one cannot do any melocho – that would indeed be a lack of halachic awareness, but it is not (from memory) the standard case of ‘helem shabbos’. Rather, the standard case is that one forgot that the day was Shabbos – he thought it was a weekday. (Friday, Sunday, whatever.) So, this seems to me to be precisely a case of misasek! How can this be considered shogeg? I know at other times in the seventh perek of Shabbos and maybe elsewhere, the terms are used interchangably – but this is not semantics here; this is a question of halacha. How can one have to bring a korbon in this case? Why is this a seeming exception to the rule of misasek?