Spell checker doesn't flag some transpositions, missing letters, etc. Form From, There, Their, They're, You're Your, Time, Tim, transmission transition, on and on.
I find without someone else to read it, you need to let it rest overnight then read it carefully the next day. I had a simple transposition that was caught by a lawyer and I corrected and looked at my template for years prior and it was repeated for years in my template. I may have read it at least 100 times. It can be difficult. In writing a book, I hired a retired teacher to proof read the book and after she did and found a few errors, I then found a couple more by simply reading my own book again.