The Gemoro on Shabbos 6b (2/3 down) brings a statement from the Megillas Sesorim, that "eino chayav ela achas", then asks on it from a Mishna, and amends the quote.
Why couldn't the Gemoro say that the Megillas Sesorim speaks about Shig'gas Shabbos, so that he is Chayav once for several melochos?
Meir Eliezer Bergman, Manchester UK
The Gemara here asks why the Mishnah (73a) mentions the number of Melachos. The Gemara answers (according to the explanation of Rashi, DH Chayav) that the reason why the Mishnah gives the number is to teach the maximum number of Chata'os that a person can be liable to bring as a result of one He'elem.
Therefore, it follows that since Isi ben Yehudah also mentions how many Melachos there are, his intention is also to teach the maximum number of Chata'os a person could be liable at once. Consequently, he could not have been discussing Shigegas Shabbos (that the person forgot that today is Shabbos), because if so the person would be liable only for one Chatas, and it would not have been necessary for Isi to mention the number 39.
Kol Tuv,
Dovid Bloom