As an occasional writer, I find myself making these sorts of errors for a reason that sounds odd. I write what I am thinking, then when I go back over a paragraph I find my brain feeding back what I MEANT to write not what I DID write. This is why it is a good idea to proof read after some time, maybe a day or two, has passed so you are less likely to gloss over the errors you made.
When these errors occur, try not to find full fault with the writer. After all, the editor should be picking it up as well.
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