Isaiah 51:12 (King James Bible) who art thou that thou shouldest be afraid of a man that shall die and of the son of man which shall be made as grass
Here in 2 Nephi 8:12, the Book of Mormon text has simply “afraid of man”, while the King James Bible has “afraid of a man”. Since there is no indefinite article in Hebrew, the use of the a in an English translation is a question of interpretation. The use of man alone offers a better parallel with the following “the son of man” since it does not read “the son of a man” in the King James text. Of course, it is also quite possible that the indefinite article a was accidentally skipped during the initial transmission of the Book of Mormon text. But at least it was not dropped when 𝓞 was copied into 𝓟 since 𝓞 is extant here and reads “afraid of man”.
Related to this issue is the repeated occurrence of the relative pronoun which in this passage, unlike the King James text which first has that in italics (“that shall die”) and then which, also in italics (“which shall be made as grass”). Of course, either which or that is an appropriate translation. But the parallel use of the same relative pronoun which in the Book of Mormon text gives further support to the parallelism of man and the son of man in the same passage. (For further discussion regarding which versus that, see under which in volume 3.)
Summary: Accept the reading of the original manuscript in 2 Nephi 8:12 (“afraid of man”); the omitted indefinite article a parallels the following “of the son of man”; the repetition of the relative pronoun which in the Book of Mormon text further increases the parallelism of this passage.