(a) In a case where a Sa'ah of Terumah fell into a hundred Sa'in of Chulin, and after the owner ground them (see Tosfos Yom-Tov [in one go]), there are less than a hundred and one Sa'ah, the Mishnah rules that the flour is nevertheless permitted to a Zar - because if the Chulin diminished, so did the Terumah.
(b) In a case where the Sa'ah fell into less that hundred Sa'in and after grinding, the owner discovers that there are now a hundred and one Sa'in - the mixture remains Asur, because here too, the Terumah increased just as the Chulin did.
(c) The mixture will be permitted - if the Chulin are of a better quality than the Terumah (see Tosfos Yom-Tov and Tosfos Rebbi Akiva Eiger, as to why this clause is omitted from the Reisha).
(d) It is even permitted to grind the mixture Lechatchilah (see Tosfos Yom-Tov).
(a) Finally, the Tana rules that in a case where, after a Sa'ah of Terumah fell into less than a hundred Sa'in of Chulin, a bundle of Chulin 'falls in', bringing the total of Chulin to a hundred and one - it is permitted provided the Chulin fell in inadvertently, but Asur if the owner threw it in on purpose ...
(b) ... because the Chachamim penalized the owner for being Mevatel an Isur Lechatchilah (and this extends to all Isurim min ha'Torah).
***** Hadran Alach 'Sa'ah' *****