Heb 10:5 KJV - "Wherefore when he cometh into the world, he saith, Sacrifice and offering thou wouldest not, but a body hast thou prepared me:"

Psalm 39:6 LXX (LXX numbering differs, but this is the same verse as below) - "Sacrifice and offering Thou hast not desired, but a body Thou hast prepared for me..."

Psalm 40:6 KJV (Based on Masoretic Text) - "Sacrifice and offering thou didst not desire; mine ears hast thou opened: burnt offering and sin offering hast thou not required."

Notice that our JKV Bible, based on the Hebrew Masoretic Text, does not agree with the quote from Hebrews, while the LXX does contain the quoted phrase. This is only one of a number of examples where the LXX contains a NT quote, while the JKV, based on the Hebrew Masoretic Text, leaves it out. How weird is that? Actually, the OT in most Bible translations is based on the Hebrew Masoretic Text. In this case, the NT is quoting the Psalm in support of the pre-existence of Messiah and his entry into the world in a real human body prepared beforehand by YHWH. This is a teaching held by the early believers regarding Y'shua but denied by the Jewish establishment of Messiah's day as regards Y'shua. Other verses where the Hebrew Masoretic Text leaves out NT quotes from the OT which are included in the LXX are similarly proof texts that were problematic for the unbelieving Jews.

Does the fact that the Greek LXX agrees with the NT writer's quotes while the Hebrew Masoretic Text does not mean that the NT writers were quoting the Greek LXX? Not necessarily. It could just as easily mean that the Hebrew OT Bible then in use by the Apostles DID contain the now-problematic verses. In fact, the 1st century Hebrew Bible has been proven to contain said verses - thanks to the Dead Sea Scrolls. Notice this quote regarding the Dead Sea Scroll scholarship on this.

With the discovery of the Dead Sea Scrolls in the mid twentieth century many examples have been recovered of the Old Testament in Hebrew from the time of Christ and the Holy Apostles and earlier. Scholarship during the past half century based upon these Dead Sea discoveries has revealed a close agreement between the LXX and pre-Masoretic Hebrew texts. In a review of some of this scholarship, Hershal Shanks notes that ”…many Hebrew texts [are available] that were the base text for Septuagintal translations…”. Further he notes that what ”…texts like 4QSama show is that the Septuagintal translations are really quite reliable” and ”…gives new authority to the Greek translations against the Masoretic text”. Quoting Frank Moore Cross (a co-author of the book under review), Hershal continues ”We could scarcely hope to find closer agreement between the Old Greek [Septuagintal] tradition and 4QSama than actually is found in our fragments”.  [http://en.orthodoxwiki.org/Septuagint#Variations_with_the_
Masoretic_Text_.28MT.29 and Hershal Shanks, 4QSama - The Difficult Life of a Dead Sea Scroll, Biblical Archaeology Review, Vol 33 No 3, May/June 2007, pp66-70.]

Clearly, if the much older Hebrew OT of the Dead Sea Scrolls contains the NT quoted phrases that are left out from the later Hebrew Masoretic Text it can only mean one thing. In their denial of Y'shua the Messiah, the Rabbinic Jews altered the Scriptures in those problematic passages to obscure the truth that Y'shua is the Messiah!