Rashi has a very curious comment on the verse in last week's Torah portion, "You shall put these words of mine in your heart and soul, and you shall bind them as a sign on your hand, and fix them as an emblem on your forehead" (Deut. 11:18). Rashi comments:
Even after you are exiled, you must distinguish yourselves by performing the commandments. Set the tefillin in place, and affix the mezuzos, so that they do not seem novel when you return. Similarly, it is said, "Establish marks of distinction for yourself" (Jer. 31:20).Rashi seems to be seeing that one might have thought that the commandments of tefillin and mezuzah only apply in the land of Israel. However, they apply also in exile so that they don't seem novel when we return to Israel. The Divrei Eliyahu quotes the Vilna Gaon as pointing out that this seems to contradict an explicit Gemara. The Gemara (Kiddushin 36b) states that any commandment that is dependent on land only applies in Israel, but all other commandments apply equally inside and outside of the land of Israel. If that is the case, how can Rashi state that tefillin and mezuzah, which are obligations on the individual and not connected to land at all, do not really apply in exile?
Rather, suggests the Vilna Gaon, there is a scribal error in our text of Rashi. It might have been written in earlier texts the abbreviations h"t a"m which a scribe mistook as meaning hanihu tefillin asu mezuzos (set the tefillin in place and affix the mezuzos) but it really stood for hafrishu terumos isru ma'asros (remove the terumah and the ma'aser, referring to land-based obligations that should not, ideally, apply in exile.
R. Ya'akov Tzvi Mecklenburg was evidently unaware of this commentary of the Vilna Gaon because in his Ha-Kesav Ve-Ha-Kabbalah (Deut. 11:12), he offers a similar explanation. However, he points out that the only early attestations to Rashi's text -- the Ramban and Rabbenu Bahya -- agree with the printed text we have. That certainly argues against this proposed textual emendation. He then proposed a different solution, utilizing the metaphor of the Sifrei that served as Rashi's basis, that of an estranged who wife returns to her father's home but continus to dress up like a wife so that it is not new to her when she reconciles with her husband. R. Mecklenburg explains that if the estranged wife refrains from dressing up, she is showing that she gives up on her husband. However, if she continues to dress up, she shows her continued connection with her husband and her expectations for the future. So, too, Jews must keep the commandments in exile to demonstrate their continued connection to God and hope for the ultimate redemption.
Earlier, the Maharal from Prague, in his Gur Aryeh (ad loc.), explained Rashi's intention as being that if we were exempt from commandments in exile, when we return from Israel God would need to give us the Toah a second time. Because that is an unacceptable option, we must retain our connection to the first giving of the Torah by following its commandments even in exile.
Later, R. Avraham Binyamin Sofer, in his Kesav Sofer (ad loc.), suggests a different explanation that emphasizes tefillin and mezuzah. Those commandments, more than any other, are intended to serve as a sign to other nations. If we are living peacefully in our land, then the signs serve the divine purpose of spreading the glory of God. But if we are living in exile from our land, wandering throughout the world, then the sign serves the exact opposite and even lowers the esteem of God in the eyes of the nation. Therefore, one might have thought that these commandments do not apply in exile. However, they do, so that they do not seem new to us when we return to Israel. (See also the introduction to Responsa Oneg Yom Tov)
My point here is that conjectural emendations are not a good idea. If even the Vilna Gaon could suggest an emendation that was later refuted, then none of us should even dare to venture into that territory. If you are examining texts, stick to the manuscripts of which we have now been blessed with a plethora.