In the ASV of Hebrews we have the following:

"And when he again bringeth in the firstborn into the world he saith, And let all the angels of God worship him. And of the angels he saith, Who maketh his angels winds, And his ministers a flame of fire" (Heb 1:6-7, ASV)

From the English of the Masoretic text, notice the preceding underlined is missing.

"Rejoice, O ye nations, with his people: For he will avenge the blood of his servants, And will render vengeance to his adversaries, And will make expiation for his land, for his people." (Deut 32:43, ASV)

"Rejoice, O ye nations, with his people: for he will avenge the blood of his servants, and will render vengeance to his adversaries, and will be merciful unto his land, and to his people." (Deut 32:43, KJV)

Hebrews 1:6-7 is quoted from the LXX, and the Masoretic deleted that important line.

"Rejoice, ye heavens, with him, and let all the angels of God worship him; rejoice ye Gentiles, with his people, and let all the sons of God strengthen themselves in him; for he will avenge the blood of his sons, and he will render vengeance, and recompense justice to his enemies, and will reward them that hate him; and the Lord shall purge the land of his people." (Deut 32:43, LXXE)

Then again in Hebrews we have:

Hebrew 10:5 "Therefore, when He came into the world, He said: 'Sacrifice and offering you did not desire, but a body you have prepared for me'..."

The underlined is missing in the KJV based on the Masoretic text.

"Sacrifice and offering thou didst not desire; mine ears hast thou opened: burnt offering and sin offering hast thou not required." (Ps 40:6, KJV)

It is found in the LXX, from whence it was quoted.

(39:6) Sacrifice and offering thou wouldest not; but a body hast thou prepared me: whole-burnt-offering and sacrifice for sin thou didst not require. (Ps 40:6, LXXE)

Did the Masoretic text intentionally drop these important phrases? Would we do well to pay more attention to the LXX in the future?

The preceding came from this rather long article: http://www.setterfield.org/Septuagint_History.html

The latest English translation of the LXX is available online here: http://ccat.sas.upenn.edu/nets/edition/