Regarding the possibility of deceiving gedolim and the fact that they are not infallible - this should be obvious. In fact this was stated by the spokesman for the Aguda - Rabbi Shafran available on Wikipedia and other places
Da'at Torah is not some Jewish equivalent to the Catholic doctrine of papal infallibility. Not only can rabbis make mistakes of judgment, there is an entire tractate of the Talmud, Horiut, predicated on the assumption that they can, that even the Sanhedrin is capable of erring, even in halachic matters.
What Da'at Torah means, simply put, is that those most imbued with Torah-knowledge and who have internalized a large degree of the perfection of values and refinement of character that the Torah idealizes are thereby rendered particularly, indeed extraordinarily, qualified to offer an authentic Jewish perspective on matters of import to Jews - just as expert doctors are those most qualified (though still fallible, to be sure) to offer medical advice.
Rabbi Bechhofer has written a fascinating article regarding the deception of gedolim concerning a forgery of the Yerushalmi.
The Talmud Yerushalmi on Kodashim
Rabbi Yosef Gavriel Bechhofer, Editor Or Shmuel, Rosh Kollel, Frumi Noble Night Kollel of Hebrew Theological College.
It seems clear from the Rishonim that they had access to the Talmud Yerushalmi on Seder Kodashim, In the introduction to his commentary on the Mishnah, the Rambam states explicitly that on the first five sedarim, both the Talmud Bavli and Talmud Yerushalmi are extant. During the course of time, however, the Yerushalmi on the entire seder of Kodashim was lost, and for several hundred years no manuscript on this seder was known to exist. (See the introduction of Rabbi Mordechai Zev Segal of Lvov to the Zhitomer  edition of the Talmud Yerushalmi.)
In the year 1907, however, a mysterious person suddenly appeared in Hungary, calling himself Rabbi Shlomo Yehuda Algazi-Friedlander. Rabbi Algazi-Friedlander j published what he claimed to be the Yerushalmi on tractates Chullin and Bechoros, thus instigating a battle royal amongst the Gedolei Hador. A personal account of this chapter in the history of the Talmud was written by Rabbi Yekusiel Yehuda Greenwald of Columbus, Ohio, and printed in the Sefer Hayovel of HaPardes (1953), Here is a synopsis of the story,