"Just As It Is Written"?

Just as it is written,
"Behold, I lay in Zion a stone of stumbling and a rock of offense,
And he who believes in him will not be disappointed."
- Romans 9:33 (New American Standard Bible/NASB)

But that's not "just as it is written."

In Romans 9:33, the Apostle Paul combines two Scripture verses, Isaiah 28:16 and Isaiah 8:14, to come up with his quote. Here is how those verses read:

Isaiah 28:16:

Therefore thus says the Lord GOD,
"Behold, I am laying in Zion a stone, a tested stone,
A costly cornerstone for the foundation, firmly placed.
He who believes in it will not be disturbed." (Hebrew: NASB)

therefore thus says the Lord,
See, I will lay for the foundations of Sion
a precious, choice stone,
a highly valued cornerstone for its foundations,
and the one who believes in him will not be put to shame. (Greek: New English Translation of the Septuagint/NETS)

Isaiah 8:14:

"Then He shall become a sanctuary;
But to both the houses of Israel, a stone to strike and a rock to stumble over,
And a snare and a trap for the inhabitants of Jerusalem." (Hebrew: NASB)

If you trust in him, he will become your holy precinct, and you will not encounter him as a stumbling caused by a stone nor as a fall caused by a rock, but the house of Iakob is in a trap, and those who sit in Ierousalem are in a pit. (Greek: NETS)

While one can see from the English translations that Paul did not seem to exclusively follow either the Hebrew text or the Greek (Septuagint) version, the following shows the relationship between the Septuagint translation of these verses and Romans 9:33: blue from Isaiah 28:16, red from Isaiah 8:14, and green the part of one word that may have come from either verse:

Isaiah 28:16: δια τουτο ουτως λεγει Κυριος Ιδου εγω εμβαλω εις τα θεμελια Σιων λιθον πολυτελη εκλεκτον ακρογωνιαιον εντιμον εις τα θεμελια αυτης και ο πιστευων επ' αυτω ου μη καταισχυνθη

Isaiah 8:14: και εαν επ' αυτω πεποιθως ης εσται σοι εις αγιασμα και ουχ ως λιθου προσκομματι συναντησεσθε αυτω ουδε ως πετρας πτωματι ο δε οικος Ιακωβ εν παγιδι και εν κοιλασματι εγκαθημενοι εν Ιερουσαλημ

Romans 9:33: καθως γεγραπται, Ιδου τιθημι εν Σιων λιθον προσκομματος και πετραν σκανδαλου, και ο πιστευων επ' αυτω ου καταισχυνθησεται.

Here are the verses in table form for comparison:

Isaiah 28:16	Isaiah 8:14	Romans 9:33
δια τουτο ουτως λεγει Κυριος	και εαν επ' αυτω πεποιθως ης	καθως γεγραπται,
Ιδου εγω εμβαλω εις τα θεμελια Σιων		Ιδου τιθημι εν Σιων
	εσται σοι εις αγιασμα και ουχ ως
λιθον	λιθου προσκομματι συναντησεσθε αυτω ουδε ως πετρας πτωματι	λιθον προσκομματος και πετραν σκανδαλου,
πολυτελη εκλεκτον ακρογωνιαιον εντιμον εις τα θεμελια αυτης	ο δε οικος Ιακωβ εν παγιδι και εν κοιλασματι εγκαθημενοι εν Ιερουσαλημ
και ο πιστευων επ' αυτω ου μη καταισχυνθη		και ο πιστευων επ' αυτω ου καταισχυνθησεται.

As you can see, Paul sandwiches part of the middle of Isaiah 8:14 between some of the beginning and most of the end of Isaiah 28:16.

Thus, as opposed to quoting Isaiah "just as it is written," what Paul wrote does not seem to fully agree with either the Greek text or the Hebrew text of these verses, and his wording also has some morphological differences with what was "written."