Chisurei Mechasra — Eruvin 42a
תלמוד בבלי מסכת עירובין דף מב/א
וחסורי מחסרא והכי קתני
In the Introduction to the Pe'as HaShulchan, the author remarks about his Rebbe, the Gra, that his opinion was that any time that the Bavli interprets a mishnah with a chisurei mechasra — i.e., the introduction of "missing words" into the mishnah — in fact, the mishnah was not missing any words at all. Rather, R' Yehuda HaNasi, in his edition of the mishnah accepted the opinion of one Tanna, and edited the mishnah accordingly, and according to that Tanna, nothing whatsoever is missing. The Gemara, however, was inclined to accept the opinion of another Tanna, and added the missing words so as to reconstruct the mishnah accordingly.
Perhaps the Gra's assertion may serve to explain why the tool of chisurei mechasra is not employed by the Yerushalmi (Chomas Yerushalayim, Sha'ar §7).
The Shelah HaKadosh (Torah She'B'Al Peh, Klal §30), however, cites She'airis Yosef in the name of R' Matisyahu of France, who said that when the Gemara employs a chisurei mechasra there is nothing actually missing — as this would impute imperfection to the mishnah. Rather, R' Yehuda HaNasi found it necessary to make the mishnayos as terse as possible, and used the absolute minimum number of words possible while still making his meaning clear. A chisurei mechasra adds the words necessary for even the meanest intellect to comprehend the mishnah, but great scholars were always able to divine the meaning of the mishnah even without the additional words.
In a similar vein, notes the Shelah, the Ramban (Sefer HaBitachon) writes that there are many pesukim in Tanach in which we must add a word or two so as to understand the verse — but not because of any shortcoming in the text, but in a shortcoming of our appreciation of the language of the Scripture.
Teshuvos Melamed L'Ho'il (3:61), however, demonstrates that there are some places in which the chisure mechasra adds words which were not implicit in the original text of the mishnah, and that Rashi (Bava Metzia 114b) writes, concerning a chisurei mechasra there, that the Tanna forgot, missed and skipped several words.