First off, since I'm a geek, here are the two quotes straight from the script:
LUKE You're gravely mistaken. You won't convert me as you did my
[The Emperor gets down from his throne and walks up very close to
Luke. The Emperor looks into his eyes and, for the first time, Luke
can perceive the evil visage within the hood.]
EMPEROR Oh, no, my young Jedi. You will find that it is you who are
mistaken...about a great many things.
Technically, the script is right. The key factor in determining tensing of the verb following the word "who" is the nature of the subject noun placed immediately before it. The use of "who" creates a compound object that is then applied to a new subject. Examples:
You like ice cream.
You are the one who likes ice cream.
It is you who like ice cream.
From first to second sentence, the subject applied to the verb "like" changes, from the second-person "you" to the third-person "the one". This means that the verb of this clause must change with the subject. The original subject "You" is then termed with "are" to identify the subject of the full sentence as the compound object, a singular entity that likes ice cream. The connotation subtly changes between the first and second sentence; the second sentence's compound object infers that the subject is select in that identification; that of the group, only one likes ice cream.
The third sentence flips it back around again; while still having two subjects, the subject of the main sentence is now the third-person "it", and the subject of the compound object is "you". The verb is thus conjugated in the second person. The connotation is the same; there is one person who likes ice cream among the group, and the speaker is identifying that person.
The use of "who" or "that" to form a compound object is not new. From the Bible (Isaiah 55):
(NIV) Come, all you who are thirsty, come to the waters; and you who have no money, come, buy and eat!
(AKJV) Ho, every one that thirsts, come you to the waters, and he that has no money; come you, buy, and eat.
I picked two translations that use relatively modern speech, but the link has most English translations (didn't see TEV or NRSV, but I digress).