The trouble is that mistakes are smarter than us and know to diguise themselves. In my experience as a journalist I discovered that it's better to wait a little before proofreading your article. Take a break, go out and smoke, then come back and read it again. If I re-read the post right-away it looks just perfect, I don't see anything wrong.
I remember one time when I wrote Louis XIV instead of Louis XVI. Nobody noticed until a furios reader called and demanded to know what kind of idiots worked there?