The gemara asks why the Mishna at the end of Yoma refers to eating on Yom Kippur as being "assur" and not "chayav", as one who eats on Y"K is chayav kareis. The gemara explains that the Mishna is dealing with a case where one ate a chatzi shiur which, according to Reish Lakish, is only an issur derabbanan but not a chiyuv kareis.
What is amazing is that the Yerushalmi at the beginning of the 6th perek of Terumos presents Reish Lakish as holding exactly the opposite view. According to the Yerushalmi, although Reish Lakish in other areas holds that chatzi shiur is only an issur derabbanan, davka by Yom Kippur he agrees with R' Yochanan that chatzi shiur is an issur d'oraysa.
The reasoning of the Yerushalmi actually seems to be as follows: When the Torah uses the term "achila" it implies a minimum shiur of a k'zayis. However, the Torah never uses the terms "achila" with respect to the prohibition of eating on Y"K -- it uses the term "inuy". Even eating the smallest amount of food diminishes the feeling of inuy.
What are we to make of these two versions of Reish Lakish completely at odds with with each other? The Tziyun Yerushalayim on the Yerushalmi quotes R' Ya'akov Emden as offering a creative way to reconcile the two. According to the Mishna, the chiyuv kareis for eating on Yom Kippur applies only if the amount of a k'koseves hagasah, a large date (larger than a k'zayis), is eaten. R' Y"E suggests that perhaps there are actually two levels of chatzi shiur. If less than a k'zayis was eaten on Y"K, both the Bavli and Yerushalmi agree that Reish Lakish would hold that the issur is only derabbanan -- since this amount does not constitute what the Torah usually calls achila, there is no Biblical chiyuv. The Yerushalmi Terumos is adding an additional chiddush that applies only where one ate more than a k'zayis on Yom Kippur but less than a k'koseves hagasah, i.e. less than the amount that would generate a chiyuv kareis. Although technically this achila can also be called a chatzi shiur with respect to Yom Kippur, since this achila surpasses the normal threshold for what constitutes achila in other areas, even Reish Lakish would hold it is Biblically prohibited on Yom Kippur as well.