Various types of m'lacha (labor) are forbidden on the sabbath, and the punishment is a particular form of death called kares (if the perpetrator doesn't get killed by a human-run court). However, if the act was done in sh'gaga (error), the perpetrator need only repent and bring an animal offering to the Temple in Jerusalem. The Talmud Bavli, Shabas 69 amud 1, cites a dispute between Rabi Yochanan and Resh Lakish as to what is considered a sh'gaga in this regard:
Rabi Yochanan said: whereby he erred about kares [thinking there is none at stake] — even though he was deliberate about the prohibition. [That is, even if the perpetrator remembers the act he's doing is prohibited, but has forgotten about the kares entailed, he won't get kares and needs to offer the animal.]
And Resh Lakish said: [it's not sh'gaga] until he errs about the prohibition and the kares. [That is, he must have forgotten not only about the kares entailed but about the prohibition altogether for the act to be considered to have been performed in sh'gaga.]
Very well. Further along, there's an attempt to prove Rabi Yochanan correct. The Talmud cites a mishna as implying that if one does all thirty-nine categories of m'lacha in one instance of error, he must offer thirty-nine animal offerings. The Talmud then comments that the error in this case must be one of sh'gaga with respect to each m'lacha, and not one of forgetting about the sabbath altogether (because we know from elsewhere that, in the latter case, one would offer only one animal). That is, this person who is bringing thirty-nine animals to the Temple remembered, when he did his crime, about the sabbath — but erred about every single one of the categories of m'lacha.
Consider the oddity of this case. He knew about the sabbath, but — according to Resh Lakish — forgot that planting is forbidden, that dyeing is forbidden, that kneading, that weaving, that shearing are forbidden, and he did every one of those and thirty-four more, and must therefore offer thirty-nine animal offerings.
Is this even possible? What, after all, is "knowing about the sabbath" if not knowing about at least one of its prohibitions?
That last is the very argument the Talmud uses to prove Resh Lakish wrong.
This is how it does it:
According to Rabi Yochanan…, one can find [a case where the person forgot every m'lacha but remembered the sabbath]: for example, that he knew about the prohibition of sabbath [but not the kares]. But according to [Resh Lakish], how did he [forget every m'lacha but] know that it was the sabbath?
Now here's my question: I don't understand that "According to Rabi Yochanan…" bit. According to Rabi Yochanan, just as according to Resh Lakish, we're looking for a case where the person had sh'gaga with respect to each m'lacha but not with respect to the sabbath altogether. But according to Rabi Yochanan this sh'gaga can be forgetting about the kares alone. So he had sh'gaga about each m'lacha, forgetting it has kares and perhaps forgetting it's prohibited altogether, but not about the sabbath. No sh'gaga about the sabbath! That means he didn't forget even about the kares. He knew there was kares for a sabbath violation, but forgot about every m'lacha. How can that be? The same way the case is seemingly impossible according to Resh Lakish, isn't it seemingly impossible according to Rabi Yochanan? What, after all, is "knowing about kares for the sabbath" if not knowing about kares for at least one of its prohibitions?
 It immediately refutes the argument by finding another aspect of the sabbath one can know about, besides one of the prohibitions.