The Bi-ur Halacha [סי' תרי"ח] cites a tshuva of the Binyan Tziyon [of Rav Yaakov Etlinger] who says that if a choleh ate a half of a shiur on Yom Kippur in order to save his life and doesn't need to eat the other half, he is chayav kares if he eats more and completes the shiur.
This is scandalous, according to Mori V-rabbi Shlita. He didn't eat a whole shiur of something assur, only half a shiur [the second eating]. The first eating was bi-heter gamur! So how can he be chayav kares???! If someone was a goy and ate a half of a kzayis on Yom Kippur, then converted and ate the other half - is he chayav?? Of course not! [R' Genechovski ztz"l].
[Also, if you look at the tshuva of the Binyan Tziyon, as I did, you will see that he doesn't say what the Biyur Halacha says he did. Wonder of wonders].
[Heard at last night's weekly daf yomi bi-iyun shiur.]
See Li-horos Nosson [5/38], Rav Z'olti's article in the Moriah journal [Av 5735], Avnei Shoham [R' Shlomovitz Simman 82] and much much more, including this.
לזכות ר' משה גבריאל בן יהודית לברכה והצלחה בכל מעשי ידיו