This is one for anyone who's read with chagrin a carefully spell-checked text, only to discover that some of the words have got all the letters in the right order, but don't mean what you intended. My favourite is from my novel After Surfing Ocean Beach.
At one point, the main male character was named Brian, but near the end of the writing process I changed it to Rob. We were at the page-proof stage when the copy editor sent me a frantic e-mail: "Who is this thing called Brain?"
It was, of course, Brian but spelled wrong. When I'd done my find-and-replace to change his name, the misspelling wasn't changed. It takes sharp eyes and careful reading to get it right!