In the Septuagint source text of Isaiah 7:16, it says:
But, before the boy can know good from bad;
By resisting wicked persuasions,
He will choose to do what is right.
Then the land of two kings of which you’re afraid
Will be totally abandoned!
There are three things to notice:
First, the words don’t make sense and are contradictory. If the child doesn’t know good from bad, how can he resist wickedness and choose what’s right?
The second issue is that these words do not appear in the Hebrew Masoretic text, nor the later Aramaic translation of Isaiah, nor in the Dead Sea Scrolls (1Q and Q460). Why not?
Thirdly, the words that make no sense are suspiciously (almost) identical to several words from the previous verse, namely, “wicked, he will choose good” (πονηρία εκλέξασθαι το αγαθόν).
Take a look:
Verse 15: βούτυρον και μέλι φάγεται πριν η γνώναι αυτόν προελέσθαι πονηρά η εκλέξασθαι το αγαθόν
Verse 16: διότι πριν η γνώναι το παιδίον αγαθόν η κακόν απειθεί πονηρία εκλέξασθαι το αγαθόν
Or, comparing just the very similar parts:
πονηρά η εκλέξασθαι το αγαθόν
πονηρία εκλέξασθαι το αγαθόν
Or, in our alphabet:
ponērá ē ekléxasthai to agathón
ponēría ekléxasthai to agathón
This indicates that the scribes who copied the Septuagint text could have easily repeated, by accident, those words and transposed them into the next verse.
The only word from the confusing portion that does not appear to be duplicated from the verse above, is απειθεί (meaning “resisting”). However, later scribes may have added this to clarify or 'correct' the spurious statement, in a futile attempt to clear it up. Since without it, it would sound like the child is wicked!
While this may sound speculative, repeating some words from the immediate context is a very simple error for a scribe. It’s also an error that is unlikely to go corrected, since scribes would always (very sensibly) fear removing text - just in case it is genuine and they have just failed to understand it somehow. After all, if scribes decided to remove every bit of prophecy that they personally failed to understand, that might mean removing a lot!
However, today we can compare the LXX text with the Hebrew Masoretic and the Dead Sea Scrolls with just a few clicks of the mouse. And what do we see? That the words don’t appear there. That, plus a convenient nearby source for duplication, all adds to the possibility that it’s just an accidental scribal duplication. Finally, after removing the words, the passage now make sense, a contradiction disappears, and the flow is unharmed.
So while we may be wrong, it looks like a simple scribal error.
Very different in Aramaic!
As a side note, it is quite interesting that the later 1st century CE Aramaic translation of Isaiah chapter 7 is very similar to our other sources, except for this verse! It reads completely differently, as this:
For the sake of a New Covenant, you shall be informed to prepare the way and to build a column to tie the two kings. Keep the land that you are exalted in from before the two kingdoms.
ܡܵܪܝܵܐ ܢܲܝܬܹ̇ܐ ܥܠܲܝܟ: ܘܥܲܠ ܥܲܡܵܟ݂: ܘܥܠ ܒܹܝܬ̣ ܐܲܒ̣ܘܼܟ݂: ܝܵܘ̈ܡܵܬ̣ܵܐ ܕܠܵܐ ܐܸܬ̣ܵܘ ܡ̣ܢ ܝܵܘܡܵܐ ܕܐܲܥܒܲܪ ܠܐܲܦܪܹܝܡ ܡ̣ܢ ܝܼܗܘܼܕ݂ܵܐ ܡܲܠܟܵܐ ܕܐܵܬ̣ܘܿܪ.
Why is it so different? We have no idea!