Armstrong said immediately after the 1969 landing that he had been misquoted. He said he actually said, "That's one small step for `a' man."
In 2006, a computer analysis found evidence that Armstrong said what he said he said.
Peter Shann Ford, an Australian computer programmer, ran a software analysis looking at sound waves and found a wave that would have been the missing "a." It lasted 35 milliseconds, much too quick to be heard. The Smithsonian's space curator, Roger Launius, looked at the evidence and found it convincing.
Remembering Neil Armstrong… When an American Man Walked on the Moon