In Heb 5:7-8, we read the following:
Who in the days of his flesh, when he had offered up prayers and supplications with strong crying and tears unto him that was able to save him from death, and was heard in that he feared; though he were a Son, yet learned he obedience by the things which he suffered.
Interestingly, in spite of the context showing the text to be speaking of Jesus, perhaps due to the ambiguity introduced by v. 6, Joseph Smith believed that this passage was a parenthetical comment about Melchizedek, not Jesus. The NT mss for this passage reads thusly:
Note — the 7th and 8th verses of this chapter are a
pare<n>theses alluding to Melchisedec and not to
christ
In his footnotes to Hebrews, Kevin L. Barney wrote the following (p. 20 n. 200) which explains rather plausibly the reason why Joseph Smith understood this passage to be speaking of Melchizedek:
The proper antecedent to the relative is unclear. Since the immediately preceding name is “Melchizedek” at the end of the preceding v., it is natural on first blush to take Melchizedek as the antecedent. Indeed, a marginal note in a jst ms. suggests that vv. 7-8 “are a parenthesis alluding to Melchizedek and not to Christ.” The name “Melchizedek” is, however, part of a scriptural quotation. The subject of the two verbs preceding the quote is God the Father, yet it is clear from the material in vv. 7-8 that the Father could not be the intended referent. Although the jst is grammatically justified in reading the antecedent to the relative as referring to Melchizedek, the sense of vv. 7-8 requires the conclusion that those vv. are referring to Christ. Note that the 1979 lds edition of the kjv, after reporting this jst marginal comment, goes on to cross reference this material in these vv. to Jesus, not to Melchizedek. This cross referencing is correct. The confusion on this point derives from a grammatical mistake on the part of the author of Hebrews in his use of the relative without showing Jesus as the clear antecedent. Generally, modern translations render the passage in such a way as to avoid rendering the awkward relative clause into English.