Since my purchase of Codex Sinaiticus, Facsimile Edition (you can get one here), I've been exploring this great Bible's secrets for myself. Currently, I'm reading 1 Corinthians. I'm sure there will be some "nuggets" out of this. But today, I wanted to share where the scribe wrote 1:8 twice. The little marks start on line 7 with 'OC and end with IYXY'. You can also see the marks on the verse on both sides of lines 8, 9, 10, and ending on line 11. The "first" verse 8 starts on line 2 where the scribe seems to have dipped his stylus into fresh ink as OC is darker than the preceding IYXY. There are possibly two probables for the mistake: 1. The scribe here made the copy error, or 2. the exemplar (the manuscript that was being copied) already had the mistake which was probably due to the fact that both verse 7 and verse 8 end with IYXY (Jesus Christ). The scribe's eyes probably went back to the IYXY of verse 7 and copied verse 8 again knowing that the last part was IYXY.
Proof that copying by hand was difficult in the Ancient world.