I came across the textual variant at the end of Lk 12:38, 'Blessed are these'. The issue here is the presence / absence of οι δουλοι in μακαριοι εισιν οι δουλοι εκεινοι (I think οι δουλοι shouldn't be there, yet modern translations, ESV and NIV, put it in anyway). Sinaiticus has its own version and leaves out εκεινοι too, which was later added, as we can see on the second and third line:
The ink and lettering of the correction stands out as that of one of the later correctors, attractively named Cb2. Colour and style matches the interlinear addition αν a few lines further down (on top of an interlinear correction by an early corrector).
But there is something slightly off here, there seems some noise underneath. And indeed, NA27 has this down as Alef1, and so does NA28, which is more reliable when it comes to the corrections of Sinaiticus and, according to Tischendorf himself, "εκεινοι addidit A, item C". Suspicion was justified, εκεινοι of an earlier corrector was overwritten by a later corrector.
Look twice, read what others say about it, and look again.