Or HaChaim on Deuteronomy 17:8/אור החיים על דברים י״ז:ח׳

1 כי יפלא ממך דבר למשפט, "If a matter of judgment is hidden from you, etc." The apparently extraneous word למשפט is needed to remind us that the subject matter now being introduced, i.e. the "rebellious elder" who will be executed for insubordination to the Supreme Court, is applicable only to the kind of person who had himself been ordained as judge, etc. (compare Mishnah Sanhedrin 86). If the dissenting opinion was handed down as law by a student who had not yet been ordained, such a pupil is not liable to the penalty provided for in our paragraph. I have found a Baraitha in Sanhedrin 87 which derives the fact that an unordained student is free from the death penalty from an interpretation of the word כי יפלא in our verse as not meaning "if it is concealed," but "if the most distinguished one of the judges errs." If we base ourselves on this exegesis we can understand why the Torah added the word למשפט. It means that unless the dissenting opinion was actually formulated as a practical judgment, -as opposed to a learned opinion,- such as by the presiding judge of the Sanhedrin- the penalty provided in our paragraph is not applicable. This is also why the Mishnah added that the words in verse 12: "and the man who will act deliberately sinfully, etc." prove this point. Accordingly, our verse would prove that the dissenting elder is guilty of death only if he personally acts against the majority decision but not if he instructed others to do so. The word למשפט is needed therefore to also make him guilty if he instructed others to act on the basis of his dissenting ruling.

2We may also approach the verse from the following angle. כי יפלא …למשפט, "Even if the dissenting opinion was issued as an instruction to act contrary to the majority ruling of the local court, and it is proven that the majority opinion was in error, if the subject matter was one where deliberately sinning carries the karet penalty, etc., the dissenting judge will not be executed unless he repeated the same ruling in the face of the majority opinion of the Supreme Court, openly defying them." This is also the concensus of the sages in the Talmud on the application of this halachah.