I didn’t notice it when I was proofreading the page.

I didn’t notice it when I was transcluding the page.

I did notice it somewhere beneath Belgravia when I was re-reading it on my Bebook One.

Typos or, more accurately, “Scannos“, uncorrected OCR errors, are a constant problem.  At least for me.  Despite all the measures to prevent them, I still find some later on my third re-read of the material I proofread in the first place,

In many ways, proofreading is never necessarily complete.   There is always the chance that you missed something regardless of however many times you read through it.