Well, if it was inspired scripture, then why didn't they use it in all cases. I found Blogger's 11,196 (along with the full linked article) very helpful in answering questions like these. Again, it's not a matter of declaring the Septuagint to be all error, just that it is not inerrant, thus disqualifying it from being scripture. It is reasonable to assume that when the Apostles saw clear error they did not quote from it.
The Apostles considered it Scripture. They quote from it. They use is as Scriputre (OT). The number of verses where they quote verses different from those of the Pharisaical bible is astaunding, and so is the Protestant denial:
Be your own witness.
Matt. 1:23 / Isaiah 7:14 - behold, a "virgin" shall conceive. Hebrew - behold, a "young woman" shall conceive.
Matt. 3:3; Mark 1:3; John 1:23 / Isaiah 40:3 - make "His paths straight." Hebrew - make "level in the desert a highway."
Matt. 9:13; 12:7 / Hosea 6:6 - I desire "mercy" and not sacrifice. Hebrew - I desire "goodness" and not sacrifice.
Matt. 12:21 / Isaiah 42:4 - in His name will the Gentiles hope (or trust). Hebrew - the isles shall wait for his law.
Matt. 13:15 / Isaiah 6:10 - heart grown dull; eyes have closed; to heal. Hebrew - heart is fat; ears are heavy; eyes are shut; be healed.
Matt. 15:9; Mark 7:7 / Isaiah 29:13 - teaching as doctrines the precepts of men. Hebrew - a commandment of men (not doctrines).
Matt. 21:16 / Psalm 8:2 - out of the mouth of babes and sucklings thou has "perfect praise." Hebrew - thou has "established strength."
Mark 7:6-8 – Jesus quotes Isaiah 29:13 from the Septuagint – “This people honors me with their lips, but their heart is far from me; in vain do they worship me, teaching as doctrines the precepts of men.”
Luke 3:5-6 / Isaiah 40:4-5 - crooked be made straight, rough ways smooth, shall see salvation. Hebrew - omits these phrases.
Luke 4:18 / Isaiah 61:1 - and recovering of sight to the blind. Hebrew - the opening of prison to them that are bound.
Luke 4:18 / Isaiah 58:6 - to set at liberty those that are oppressed (or bruised). Hebrew - to let the oppressed go free.
John 6:31 / Psalm 78:24 - He gave them "bread" out of heaven to eat. Hebrew - gave them "food" or "grain" from heaven.
John 12:38 / Isaiah 53:1 - who has believed our "report?" Hebrew - who has believed our "message?"
John 12:40 / Isaiah 6:10 - lest they should see with eyes...turn for me to heal them. Hebrew - shut their eyes...and be healed.
Acts 2:19 / Joel 2:30 - blood and fire and "vapor" of smoke. Hebrew - blood and fire and "pillars" or "columns" of smoke.
Acts 2:25-26 / Psalm 16:8 - I saw...tongue rejoiced...dwell in hope.. Hebrew - I have set...glory rejoiced...dwell in safety.
Acts 4:26 / Psalm 2:1 - the rulers "were gathered together." Hebrew - rulers "take counsel together."
Acts 7:14 / Gen. 46:27; Deut. 10:22 - Stephen says "seventy-five" souls went down to Egypt. Hebrew - "seventy" people went.
Acts 7:27-28 / Exodus 2:14 - uses "ruler" and judge; killed the Egyptian "yesterday." Hebrew - uses "prince" and there is no reference to "yesterday."
Acts 7:43 / Amos 5:26-27 - the tent of "Moloch" and star of god of Rephan. Hebrew - "your king," shrine, and star of your god.
Acts 8:33 / Isaiah 53:7-8 - in his humiliation justice was denied him. Hebrew - by oppression...he was taken away.
Acts 13:41 / Habakkuk 1:5 - you "scoffers" and wonder and "perish." Hebrew - you "among the nations," and "be astounded."
Acts 15:17 / Amos 9:12 - the rest (or remnant) of "men." Hebrew - the remnant of "Edom."
Rom. 2:24 / Isaiah 52:5 - the name of God is blasphemed among the Gentiles. Hebrew - blasphemed (there is no mention of the Gentiles).
Rom. 3:4 / Psalm 51:4 - thou mayest "prevail" (or overcome) when thou art judged. Hebrew - thou might "be clear" when thou judges.
Rom. 3:12 / Psalm 14:1,3 - they "have gone wrong." Hebrew - they are "corrupt" or "filthy."
Rom. 3:13 / Psalm 5:9 - they use their tongues to deceive. Hebrew - they flatter with their tongues. There is no "deceit" language.
Rom. 3:13 / Psalm 140:3 - the venom of "asps" is under their lips. Hebrew - "Adder's" poison is under their lips.
Rom. 3:14 / Psalm 10:7 - whose mouth is full of curses and "bitterness." Hebrew - cursing and "deceit and oppression."
Rom. 9:17 / Exodus 9:16 - my power "in you"; my name may be "proclaimed." Hebrew - show "thee"; may name might be "declared."
Rom. 9:25 / Hosea 2:23 - I will call my people; I will call my beloved. Hebrew - I will have mercy (love versus mercy).
Rom. 9:27 / Isaiah 10:22 - only a remnant of them "will be saved." Hebrew - only a remnant of them "will return."
Rom. 9:29 / Isaiah 1:9 - had not left us "children." Hebrew - Jehova had left us a "very small remnant."
Rom. 9:33; 10:11; 1 Peter 2:6 / Isaiah 28:16 - he who believes will not be "put to shame." Hebrew - shall not be "in haste."
Rom. 10:18 / Psalm 19:4 - their "voice" has gone out. Hebrew - their "line" is gone out.
Rom. 10:20 / Isaiah 65:1 - I have "shown myself" to those who did not ask for me. Hebrew - I am "inquired of" by them.
Rom. 10:21 / Isaiah 65:2 - a "disobedient and contrary" people. Hebrew - a "rebellious" people.
Rom. 11:9-10 / Psalm 69:22-23 - "pitfall" and "retribution" and "bend their backs." Hebrew - "trap" and "make their loins shake."
Rom. 11:26 / Isaiah 59:20 - will banish "ungodliness." Hebrew - turn from "transgression."
Rom. 11:27 / Isaiah 27:9 - when I take away their sins. Hebrew - this is all the fruit of taking away his sin.
Rom. 11:34; 1 Cor. 2:16 / Isaiah 40:13 -the "mind" of the Lord; His "counselor." Hebrew - "spirit" of the Lord; "taught" Him.
Rom. 12:20 / Prov. 25:21 - feed him and give him to drink. Hebrew - give him "bread" to eat and "water" to drink.
Rom. 15:12 / Isaiah 11:10 - the root of Jesse..."to rule the Gentiles." Hebrew - stands for an ensign. There is nothing about the Gentiles.
Rom. 15:21 / Isaiah 52:15 - been told "of him"; heard "of him." Hebrew - does not mention "him" (the object of the prophecy).
1 Cor. 1:19 / Isaiah 29:14 - "I will destroy" the wisdom of the wise. Hebrew - wisdom of their wise men "shall perish."
1 Cor. 5:13 / Deut. 17:7 - remove the "wicked person." Hebrew - purge the "evil." This is more generic evil in the MT.
1 Cor. 15:55 / Hosea 13:14 - O death, where is thy "sting?" Hebrew - O death, where are your "plagues?"
2 Cor. 4:13 / Psalm 116:10 - I believed and so I spoke (past tense). Hebrew - I believe, for I will speak (future tense).
2 Cor. 6:2 / Isaiah 49:8 - I have "listened" to you. Hebrew - I have "answered" you.
Gal. 3:10 / Deut. 27:26 - cursed be every one who does not "abide" by all things. Hebrew - does not "confirm" the words.
Gal. 3:13 / Deut. 21:23 - cursed is everyone who hangs on a "tree." Hebrew - a hanged man is accursed. The word "tree" does not follow.
Gal. 4:27 / Isaiah 54:1 - "rejoice" and "break forth and shout." Hebrew - "sing" and "break forth into singing."
2 Tim. 2:19 / Num. 16:5 - The Lord "knows" those who are His. Hebrew - God will "show" who are His.
Heb. 1:6 / Deut. 32:43 - let all the angels of God worship Him. Hebrew - the Masoretic text omits this phrase from Deut. 32:43.
Heb. 1:12 / Psalm 102:25 - like a "mantle" ... "roll them"... "will be changed." Hebrew - "raiment"... "change"..."pass away."
Heb. 2:7 / Psalm 8:5 - thou has made Him a little "lower than angels." Hebrew - made Him but a little "lower than God."
Heb. 2:12 / Psalm 22:22 - I will " sing" thy praise. Hebrew - I will praise thee. The LXX and most NTs (but not the RSV) have "sing."
Heb. 2:13 / Isaiah 8:17 - I will "put my trust in Him." Hebrew - I will "look for Him."
Heb. 3:15 / Psalm 95:8 - do not harden your hearts as "in the rebellion." Hebrew - harden not your hearts "as at Meribah."
Heb. 3:15; 4:7 / Psalm 95:7 - when you hear His voice do not harden not your hearts. Hebrew - oh that you would hear His voice!
Heb. 8:9-10 / Jer. 31:32-33 - (nothing about husband); laws into their mind. Hebrew - I was a husband; law in their inward parts.
Heb. 9:28 / Isaiah 10:22 - "to save those" who are eagerly awaiting for Him. Hebrew - a remnant of them "shall return."
Heb. 10:5 / Psalm 40:6 - "but a body hast thou prepared for me." Hebrew - "mine ears hast thou opened."
Heb. 10:38 / Hab. 2:3-4 - if he shrinks (or draws) back, my soul shall have no pleasure. Hebrew - his soul is puffed up, not upright.
Heb. 11:5 / Gen. 5:24 - Enoch was not "found." Hebrew - Enoch was "not."
Heb. 11:21 / Gen. 47:31 - Israel, bowing "over the head of his staff." Hebrew - there is nothing about bowing over the head of his staff.
Heb. 12:6 / Prov. 3:12 - He chastises every son whom He receives. Hebrew - even as a father the son in whom he delights.
Heb. 13:6 / Psalm 118:6 - the Lord "is my helper." Hebrew - Jehova "is on my side." The LXX and the NT are identical.
James 4:6 / Prov. 3:34 - God opposes the proud but gives grace to the humble. Hebrew - He scoffs at scoffers and gives grace to the lowly.
1 Peter 1:24 / Isaiah 40:6 - all its "glory" like the flower. Hebrew - all the "goodliness" as the flower.
1 Pet. 2:9 / Exodus 19:6 - you are a "royal priesthood." Hebrew - you shall be to me a "kingdom of priests."
1 Pet. 2:9 / Isaiah 43:21 - God's own people...who called you out of darkness. Heb. - which I formed myself. These are different actions.
1 Pet. 2:22 / Isaiah 53:9 - he "committed no sin." Hebrew - he "had done no violence."
1 Pet. 4:18 / Prov. 11:31 - if a righteous man "is scarcely saved." Hebrew - if the righteous "is recompensed."
1 Pet. 5:5 / Prov. 3:34 - God opposes the proud but gives grace to the humble. Hebrew - He scoffs at scoffers and gives grace to lowly.
Isaiah 11:2 - this verse describes the Seven Gifts of the Holy Spirit, but the seventh gift, "piety," is only found in the Septuagint.
Ref http://www.scripturecatholic.com/septuagint.html
If any of these agree with the KJV or any current English Bible, that means you are reading Septuagint as Scriputre and not the Hebrew Bible. And I can tell you Matt. 1:23 / Isaiah 7:14 in YOUR Bible is from the Septuagint!